Your water team is already very competent with Ryu-oh and Aphrodite. Other water SSRs are still helpful, but not very necessary, and some nice ones are not available yet. So better invest on other elements, particularly wind, to cover water's weakness.

As SR, Belphegor and Kikuri-hime are quite good. With Aphrodite in the party, you won't miss Nike too much.

BTW, if following DMM version's schedule, the pick-up multiplier shall be substantially increased, from 3x to 15x IIRC. The chance of getting a particular KH is still low ofc.