I know you guys know how much swimming as a transman sucks. It is the time that we face our biggest differences between us and cis men. Our chest ain’t flat, our hips go out, it sucks. Despite that, there are options.
For binding, I would suggest having a designated binder for swimming. I say this because when you get out of the water, you’ll want to dry off and a wet binder is never a comfortable binder.
If you are out and comfortable with those around you, then obviously you can just wear a binder and go out to swim (I would suggest flavnt swim binders if you do so). If you are stealth or simply don’t want you show everyone your binder, then use a rash guard or swim shirt over your binder.
You can even use a regular T-shirt but, I wouldn’t recommend it since not only might you damage the T-shirt but it’s also insanely uncomfortable. At the beach, a lot of people wear rash guards and swim shirts. If you’re a bigger guy, wearing a shirt to the pool is pretty common as most bigger guys aren’t too keen on showing their body (sad, but it’s a reality even I felt post-op since I was still bigger). A person or two might question you but usually the statement “my skin is a bitch” is self explanatory, and even then, most people are too concerned with having fun in the pool to question what you’re wearing.
KT Tape
If you have a smaller chest, then KT Tape is also a great option for swimming (and for everyday too but mostly for swimming). KT Tape is kinesiology tape that most athletes use for joint pain/strain. That being said, when you use KT Tape, you have to be cautious as it was not necessarily designed for skin as thin as the chest. My experience with KT tape may be different from others, but this is what has worked best FOR ME. If you feel any sort of discomfort, remove the tape. KT Tape comes in rolls of strips that look like this:

Cut the strip in half to where you have two shorter strips of the same width. Fold a single square of toilet paper and apply it to where you nipple will be on the tape. The tape should fall like this.

The other end of the tape should end vertically in line with your arm pit. Do not apply more than two strips to each side and do not allow for the strips to reach your back as that is a hazard to your breathing. After applying the tape, I usually left it on for 2 days. My reasoning for this was that the adhesive was meant to be worn for more than a few hours. I didn’t have a lot of chest so there was not a lot to bind, so if you have a larger chest, listen to your body and take it off sooner than that.
When removing KT Tape, for the love of god, DO NOT REMOVE IT LIKE A BANDAID. I have seen so many guys say KT Tape is a hoax and ruins your skin because they peeled it like a bandaid. In order to prevent a cut, take a hot shower and bring in some baby oil. After about 10 minutes, apply the baby oil to tape and rub that stuff in. By this point, you should have a corner peeling. GENTLY get a hold of the tape and SLOWLY PULL YOUR SKIN FROM THE TAPE. Do not pull the tape from your skin. I don’t know why but this will hurt your skin less. After you have removed the tape, wash your chest will cold water and MOISTURIZE. Your chest is going to need it.
This seems like a lot but after a while it becomes routine.
KT Tape and duct tape are not the same. I have used duct tape. It itches like hell and irritates your skin a hell of a lot more. Do not use duct tape.
Swim Trunks
When you’re getting swim trunks, avoid horizontal lines. Just like how shirts with horizontal lines will show your chest, these designs will show your hips. You’re better off purchasing solid color or trunks with scattered designs. As a transman, I would suggest wearing a pair of performance boxer briefs underneath just to make sure your packer stays in your trunks. Wearing boxers briefs or something underneath your swim trunks is more common than you’d think. Some guys just don’t feel comfortable with wearing nothing underneath them, or as my friend put it, “I don’t wanna be in the pool and flashing y’all.” So wear some boxer briefs or regular briefs, your choice.
Swim trunks are pretty individual to the person. Some guys like them long, others like them short. And because there isn’t really anything to cover your hips, you really do just have to try one some pairs and see what you like. I can’t truly tell you what looks good and what doesn’t because honestly, my swim trunks are neon green and cargo style that reach to my knees. Any other guy would say “hell no” to these but for me, they somehow fit my body right. For other guys, shorter pairs of trunks are what make them happy. This is really up to you.
If you are packing…
If you swimming and are packing, the beware of this. Assuming you don’t have a spare dick on hand, after you change into dry clothes, you will be using the same wet packer. Because of this, you are going to have so REALLY dry your packer and your packing device (pouch, strap, underwear, etc.). I don’t mean a few pats either, I’m talking getting into it, bro. Believe me, your packer is a lot wetter than you think.
My first pool party I went to while packing was so fun but so embarrassing afterwards. I changed out of my trunks and jumped right into the party and was dancing and having a blast until I got home and I realized “oh wow my entire crotch is wet.” I did not dry my packer well enough and was just dancing around with a giant wet spot on my crotch. It looked like I pissed myself, man. So for your own dignity, dry your packer.