Kaminari can swim, and he can do it damn well. But, of course, having an electric quirk isn't exactly safe if you're in water. So he doesn't swim or go in water much, with the exception of hot springs, because who can resist those? It's kinda sad though because he actually had a dream of becoming a professional swimmer, just because of how great he was at it, but that dream was ruined once he discovered his quirk.