I've Had a few theories about potential fish that could hold the other swimwear

Shark tuna: because the lei are tropical themed Costa seems like a potential location. I have only managed to catch one of these so far though and it failed D:

Big Octopus: seeing as Octomamoth was blue and gave the blue leg pieces I thought the red octopus might provide similar results, another thing to note is although this isn't a green rated 2 star it gives me a lower synth % than the Gigashark or Octomamoth. However I have desynth'd 4 of these with no swimwear so far.

I am also interested in desynthing Frilled Sharks but have yet to find any.