Mmm...it is all according to personal preference. The less clothing you wear, the easier it is to swim around, after all.

[Poseidon might have armor on, but he doesn't wear anything long or flowing that could get caught as he swims. Their sons are young, so it's more appropriate for them to wear what they have, especially since they are the sons of a king.

Suffice it to say that means there are plenty of mermaids who swim around with their tiddy out.]