water advice huh? not an expert, but i can share some thoughts.

well, u already have a-slot debuffs covered with ryu-oh and OD extension which will make mordred seem less worth it, unless u were also goin to be usin her on varies different teams. belphy and aphrodite are pretty good as well. b-slot debuffs 12%, rage reducer from belphy n heals plus cleanse from aphrodite.

so if u were to replace someone, it should probably be nike as her heals are meh at most. i'd probably take her out for kukuri-hime because u can still keep ur def buff and had a skill that gives 15 flat burst which can be pretty nice thou it has a 8 turn cd. or u know if u end up lucking out and gettin ea or cthulu. (side note i personally never got any hime that was advertised as having a higher chance at gettin them.)

Basically in case u didn't get all that or read it. If u were swapping out one of ur team, it would be nike. (nodens for extra cleanse n kukuri-hime for 15 flat burst increase every 8turns. As for mordred, think about gettin her if u plan on using her on other teams. but if ur only goin to be usin ur water team for a while think about tryin to get d'art for snipe shot if u don't already have her. obviously the final decision is up to u.

at least that's what i think. i hope this helps.