The basics of course include paddles, buoy, bands, & fins. I've since added sim shorts and a drag suit to my bag of tricks and I'll explain what I've learned using these tools (pros and cons and when/why to use them). These are just my personal thoughts based on my own experience as well as watching/coaching others who use these tools and seeing how it affects their swimming as well. Most of the links below take you to swimoutlet.com and I have no affiliation with them it's just a simple site where I tend to order much of my swim gear.
There are lots of types of paddles. I don't own them all but I have 3 different kinds and choose to use them each under different circumstances.
1) These basic/cheap paddles by Sporti. I LOVE THEM. I use them when I'm doing long strength sets. I tend to swim a good 3-5"/100 faster when using these vs not using paddles because they allow me to pull more water than I would when just using my hands alone. I have no fear of doing very long sets using these paddles- next week I'll do 10x400's using them the whole time. And if/when the ocean is calm, I'll sometimes use them in the ocean too for up to an hour at a time. If your technique is decent you're not going to hurt your shoulders. That said, I wouldn't give a new swimmer such long sets using paddles b/c they'd fatigue too quickly, but I've watched the athletes I coach build up to being able to handle using these for longer and longer distances so it's certainly possible to build your volume with these, and as you do, you'll be a more fatigue-resistant swimmer.
Pros: They make you stronger. You might be more likely to understand what it feels like to use your lats when you pull. You get to go faster.
Cons: They tend to slow your turnover down so if you're actively working on increasing your turnover, limit your use of pads. If your technique is shitty or you choose pads too big for your current level of strength, you could hurt your shoulders. If you use them early in a swim set but don't have the strength to back them up, the rest of your swim will be crap. So if you're new to swimming, reserve the pads for toward the end of your swim and focus on power/technique. As you get stronger can challenge yourself with using them earlier on and then trying to swim fast/strong afterward. You've made it when you can do this. :)
2) Finis Agility paddles. I'm a pretty big fan of these too! I use these more for technique oriented pulling. I don't go as fast when using these b/c have to be more careful about hand placement as you enter the water (so I never really try to 'rip it' with these on) BUT since they don't have straps, you get immediate feedback about your stroke. I.e. if you do it wrong, they fall off! How's that for feedback? So I love these for new swimmers.
Pros: They give you immediate feedback about your hand placement upon entry, catch, and pull. Great for technique work. You never have to fuss with straps breaking.
Cons: If you're focusing on increasing turnover, these won't help. Not best used in a group setting b/c you're not going to go as fast with these and the goal is NOT speed so if being around others makes you want to swim FASTER, then save these for when you're alone.
3) PT Paddles. I hate these. Ha! These essentially force the fist drill on you... You cannot feel or pull any water with your hands when using these pads so you are forced to use forearms. If you lead your pull with your elbows, you literally won't go anywhere with these paddles.
Pros: Great technique tool teaching you to involve your arms other than your hands when trying to move through the water.
Cons: Might be frustrating as you will move quite slowly through the water. Personally I don't ever use these for more than a few 50's at a time, though maybe if I sucked at them less, I'd use them more. :) Rarely will I use these in a group setting.
4) Basic buoy. I used to LOVE my buoy. Now I love it a little less (read #5 below to find out why!) The buoy has a lot of great uses- mostly that it will help swimmers pop their hips up more onto the top of the water so you get a better idea of what it feels like to be in the correct position as you're moving through the water (swimming less 'uphill'). Some swimmers swim faster with a buoy (I am faster with a buoy) but others swim slower (Nalani and Lectie are faster without their buoys). This is mostly dependent on how much you use your kick for propulsion and to keep a decent body position, and how tiring that is for you to do.
Pros: Helps many swimmers be more on top of the water, which helps to get into a position to best utilize proper catch/pull b/c swimmers aren't constantly fighting to 'swim uphill'. For triathletes- saves legs for bike/run later in the day. HR will likely be lower when using buoy b/c less energy used when not kicking. For some, you'll swim faster. If you're working on increasing turnover, a buoy will help for sure. Also, if you're into drills, a buoy can help you focus on performing the drill correctly without having to worry about sinking as much.
Cons: If you use a buoy all the time, you'll feel somewhat disconnected from your lower body and possibly missing the coordination to use your kick when it's appropriate to actually use it.
5) Roka Sim Shorts. These are my newest toy and holy cats I am a giant fan! So here you get all the benefits you love about a pull buoy, but while also allowing yourself to continue to develop the neuromuscular coordination to kick. Honestly, this product is simply brilliant and has a ton of benefits. I have tested mine in both the ocean and the pool now. I'm faster with these shorts vs a buoy on short/fast swims (50's and 100's) and about the same speed as with buoy on longer swims, but I'd say I feel better using these vs my buoy because my stroke feels more complete having the option to kick at least a decent 2-beat kick. I have not had the chance to put these shorts on Lectie or Nalani to see how they fare (given that they are faster without a buoy b/c of their kick I am genuinely curious as to how they perceive these shorts? My hypothesis is that they will like them as well but no experience with this yet.)
Pros: They pop you right up into the correct body position so you have access to all your power. Can swim with high turnover but with less accumulated fatigue. You can choose to kick strong if you want the propulsion, or hardly at all if you're focusing on your pull. Because they allow you to swim faster, you can train with athletes who are faster/stronger than you and you're more likely to 'stay in the game' through the middle/end of the session because you'll be able to go off the same intervals and be more motivated to keep pushing because you haven't been 'dropped'. My athlete Sergio has been using these in our sessions and his confidence has skyrocketed over the last month because he knows that wearing these allows him to keep up better so he tries harder, for longer... which directly impacts his swim fitness in a very positive way. If you enjoy swimming more because you can hang with the faster swimmers, you're more likely to actually swim more often, which has a direct positive impact all around.
Cons: If you swim outside in the middle of the day, you'll end up with some ugly tan lines. If the water in your pool is hot, you might suffocate yourself by wearing these too. If you don't tie the drawstring tight enough you end up collecting water inside the shorts when you push off the wall (solved if you tie it tight enough). You can't race with these in non-wetsuit legal races, but I can see myself using them for races where they give a bogus 'wetsuit legal' approval because the benefit is similar to a wetsuit but without any restrictive feeling and no issues with overheating in water that is questionably wetsuit legal.
6) Finis Ultimate Drag Suit. Ok so these are pretty much the opposite of the sim shorts above! They will slow you WAY down (for me it's in the range of 10"/100 slower). Regardless, I love them for several reasons. I sometimes swim 800-1000 straight with the drag suit, but should probably start with shorter intervals like 50's and work your way up.
Pros: They FORCE you to figure out how to pull water. Because the big pockets create so much drag, you have to create enough power upfront to overcome all that drag, so these are a great teaching tool with immediate feedback about how strongly you're pulling. They also (immediately!) fix any issue you might have with 'over-gliding' because when wearing these shorts, there is no gliding. So if you're working on increasing your turnover, these will force that. Plus, when you take them off, you feel super fast! Could also use these if you're a fast swimmer and you want to train with a friend who is slower- you genuinely get to work very hard and can go off longer intervals but still get in a some great work. Sometimes we put these on Mark trying to handicap him so we can keep up better but he still gets to work very hard. :)
Cons: Sometimes you're just too flat out tired to swim with this drag suit. It does require you to work really hard, so only use it for parts of sessions and only when you've got adequate energy. Tough to use in a group setting unless everyone has one because you'll simply be too slow to make normal send-offs.
7) Bands. No link to this one because they're easily made with an old tire tube. Just tie these around your ankles and you're good to go. Lots of ways to use bands. If you use them alone, they're similar to the drag suit above but they work in a slightly different way because they create drag by causing your legs to drop. If you're looking for drag, personally I think the suit above is a better option than bands only because you still get to kick when using the drag suit. In my experience using these tools with athletes I coach, I've seen that MOST newbie swimmers are incapable of getting across the pool when they first start to try with bands only... but those same swimmers can swim with the drag suit b/c they can still use their legs to help. So if you're new, start with the drag suit then move on to bands only. That said, if you're a strong swimmer, you might be able to swim long distances just fine with bands only. For me personally, swimming bands only is easier (by a lot!) than drag suit but that is not the case for everyone. The concept is similar anyway. It's also quite common to combine bands with buoy and/or paddles. This is mostly done when you want to completely prevent kicking and focus solely on pulling up front. I also like to use bands when I'm using a buoy b/c it helps me keep the buoy in place as I'm swimming and flip-turning.
Pros: Gives immediate feedback as to the effectiveness of your pull. If you can't get across the pool with your feet tied together, this is telling, and you've got some work to do on creating power upfront. Forces higher turnover and reduces a swimmers ability to over-glide. Cheap and easily accessible tool for any triathlete. Allows a stronger swimmer to swim with a weaker swimmer at the same speeds/send-offs.
Cons: New/weaker swimmers simply cannot do it.
8) Fins. These probably don't need a lot of explanation but I'll throw out my 2cents anyway. I tend to use fins when I want to focus on overspeed work and/or get the HR up. Fins can also be used as an aid for new swimmers are they are doing drill work or simply want to feel what it feels like to move through the water quickly. Can also be used by swimmers trying to learn other strokes like butterfly as they make swimming fly 100x easier.
Pros: If you're looking to get HR up high, these will do the trick! 10x50's max sprint with fins is one of the hardest sets we ever do! Teaches a swimmer to get comfortable being uncomfortable in the water... in an environment where we are rarely (if ever!) panting breathing, use fins if you want to simulate that and teach yourself to swim through it. They can also be used to help swimmers (who pay attention) learn where they might be creating excess drag (with head or shoulders?). When you're moving really fast, you tend to be able to feel better where you're creating drag and if can fix it, will go even faster.
Cons: I find my turnover starts to slow down when I'm focusing on kicking hard with fins, so it can be challenging to get hands and feet moving quickly at the same time while wearing fins. If you have any issues with achilles, fins can irritate them further. Swimming hard with fins is very fatiguing so if you have another bike/run session later in the day, be careful about doing too much of this (unless that's your intended goal).
So there you go! Plenty of toys to play with to keep you occupied in the water... Can mix and match too for some added fun. Once, on a day when we were just sort of playing, we wondered how hard it would be to add bands to the drag suit? We all managed to make 100M of that but it was ridiculously hard. Then of course the inevitable question... Is it even possible to swim with the PT paddles + drag suit + bands?!? Nalani was brave enough to try that combo (I was not!). It was hysterical to watch. She did make it a full 100M using all 3 of those torture devices. It took her just under 3' if I recall correctly. Ha! I prefer my new sim shorts, thank you. :)