Late last night I had an idea (which may or may not be terrible or even possible). A silicone scale-patterned one piece swimsuit. Pull a tail over it and then the scales can sort of blend together. That, or you can ever create snaps on the swimsuit top and the tail to snap and hold the pieces together. OR you could even do a waist fin (like Ariel) which divides the scales up.

Wonder if those would be possible or even work. Now I'm curious to see someone try!