I feel like someone MIGHT have asked something like this in the past, but if they did it was a while ago, and I can't remember the answers nor find the old post, if it was even a thing. Sooooo...
First, we can scratch Theodore out completely. He won't be caught dead in the water...literally. The guy has a water allergy; it's not as severe as his inspiration, the Wicked Witch of the West, where he'll literally melt into a puddle if you splash half a bucket of mopwater on him, but he WILL get some pretty nasty burns. Again, he uses magic and various special items to keep himself clean. So if he's anywhere even NEAR a pool or a beach, he's sticking to the shoreline and as far as possible from the water's edge. So he probably won't even WEAR a swimsuit, just shorts and some sort of top suitable for sunny weather as he basks and sips soda or sweet lemonade or something.
Also out of the running is Maelstrom, for the opposite reason: he's a mer-person. If he's going swimming, he's just gonna go AS HIMSELF. He'll just change to his true form after diving in, no need to change or worry about that sort of stuff (never seems to be an issue for the other Octavinelle characters).
I've always liked to imagine Billy as the "trunks and nothing else" type. He goes topless when allowed, might not even wear sandals unless the sand is REALLY hot. I like to imagine his trunks are blue and have imagery of pink bunnies on them. XD I think Grit also goes topless in trunks when he goes swimming in his more human-like form; probably a simple gray in hue, maybe with some sort of pattern that almost makes them look like stone, or perhaps with a metallic sheen. Gotta show off those rock-hard abs, right? :P
Reno and Smitty would also wear trunks, but would have the reservation to also wear a t-shirt most of the time, rather than going about topless. In the case of Reno, I feel like his would probably be colored some shades of green, to match the Oogie aesthetic, so to speak. And for Smitty...not sure about the shorts, but his shirt would probably one similar to the one he usually wears anyway, colored with blue and white stripes; in some ways, he'd actually look a lot more like Smee than he already does. XD
Elias and James I think are more the full-body swimsuit types. Elias has a love-hate relationship with water, since it's actually his biggest weakness when he goes into Blot Form, but in his normal form...well, most dogs enjoy swimming. ;) I think he'd probably wear something very sleek and slim, completely enshrouding him aside from his hands, feet, and head, clinging to his curves tightly and colored either dark blue, dark purple, or perhaps black with accents/trimming of one or the other hue. James, meanwhile, probably wears something similar, but sleeveless and MAYBE with shorter leg coverage, and colored mostly red.
And then...there's Nakoda. I keep going back and forth on him. XD Part of me wants to say something similar to Elias, though in his case it's because of the "clinging to his curves tightly" part...aaaand the other part wants to just straight up say "Speedo," not so much because of how revealing it is (well...it is, but not for THOSE reasons), but mostly because I feel that would leave everyone else either flustered or EXTREMELY embarrassed at his absolute and complete lack of shame. LOL Both feel like they would fit him and his personality in different ways, and I'm not sure which I prefer for various reasons. Maybe he goes back and forth...or tries the latter, gets kicked out of the beach/pool area, and has to change into the former. I dunno. :P
