>>279638872
Koala just has a strong fascination with fishmen because of Fisher Tiger and decided to learn it, I dont believe it's something a lot of Revos actually learn. It's also unclear at this point if humans can even take advantage of the full breath of the techniques given they rely on using the water in the air which humans may or may not be able to use.
Now on the topic of it being useful against the regen, maybe, if it comes from a DF, the same way fishman karate or salt can affect DF given their relation to water/sea.