One Piece: Seaside System - Chapter 3: Marine Based -

... five originally, but he crossed off Fishman Karate. Since he highly doubted any fishman within their kingdom would happily teach him even if he did visit and the only thing that came to mind to learn it, was to head to East Blue, slaughter Arlong and his crew and force Kurobi to teach him the basics under threat of death.

Not something at all viable right now, even if Fishman Karate was one of the few abilities capable of harming Kaido due to the shockwaves one could produce with it and ...