Either they adorn seashells when they come on land, which would be an odd modesty considering they don no other clothing for their lower parts; or... their boobs are seashells!
If they're fish and not mammals would it even make sense for them to have breasts?
Like breasts are one of those exclusive to mammals things.
I mean I've seen ones where they have dolphin or dolphin-like tails instead of a fish tail in which case them having breasts makes more sense, but that's also almost never done and instead they have a fish tail.