Do it like they do butterflies - freeze the fish first! You can always eat them afterwards (just kidding). The other day I bought my wife an amphibious 35mm because she likes to snorkel. Yeah, I know I guy who does bugs with an 8x10 ... but either dead or more often stuffed into the fridge a little while first. I've done macro with 8x10, and once enlarged, have had a few precisely-focused live bugs come to view, which I wasn't aware of when the shot was made. Wish I could find an underwater housing for
my P67, but I don't do it enough to warrant the expense, and all the gaskets for those things have to
be replaced at this point in history - probably been three decades since Pentax stopped making them.