Yeah, Pirucus are the best for leveling CUL desynth, like the guy above me said ^. Also, there are three fish that desynth into Waterproof Cotton, and at least three swimsuit pieces that also desynth into Waterproof Cotton. Blue Summer Maros are generally the cheapest way to go if you have max WVR desynth and don't want to catch fish. On my server, they sell for under 200k each, and were dropping last I checked. No one wants to wear them, really...

Only one of the fish you need to desynth for Waterproof Cotton is untradeable, but it's probably the easiest to catch. I'm having a heck of a time catching Lone Rippers for my wife to desynth, since I'm the fisher and she's the CUL desynther