[Rendar]

A badges Elemental weakness, especially for one like Quaking only gives a 25% Water weakness. Which with a stamp becomes 20% Water weakness. Any coral gear they wear lowers it even further. There are about 5 water attacks in the entire game, one of which tells them before the fight you're wearing it.

Ensui ( Show )
Drown
Black Bubble ( Half Dark, Half Water Dark usually comes first so they can resist easy )
Kel ( Not enough damage to actually matter about it hitting a weakness. It'll do about 25 more damage, maybe. )
Parraeta ( Grinding Youkai )
Whirlpool ( Grinding Youkai )

Okay. So six I can think of off the top of my head. Most of which go to summoners. WIL based summoners.