why 10% buff per water character when u could use the 8% atk/nin scale for every standard

You are relying on all the water-type ninjas to survive whatever damage is incoming for the chojuros to keep that 10%-for-each buff. Against a triple hiruzen/agk/6p, this is infeasible. The 8% atk/nin scale on standard is as you said that much more versatile and less supporting-cast dependent.