In kage treasure there's mei, in jincuriki 2 there's Han and utakata, In jinchuriki 1 there's yagura these ninjas Ideally work well with water main. to get Han and utakata you need 210 seals combined for both, for yagura if you get lucky you need 50-60 seals, mei is tricky you might get her as your first rare or your second or third or you have to wait for her as your 6th rare from kage so she takes somewhere from 60-400 seal scrolls if get really bad luck. However the really best ninjas for water main are not in the treasures but in events like GNW 1010 or school girl Sakura and edo tensei Haku etc. The most common competitiveteam for water main is the mabui 1010 team but the current best in my personal opinion is a team composed of school/sailor sakura, mei, Han and water main or sailor sakura roshi and Kabuto with water main if you decide to go for roshi instead of Han.

that is not even remotely viable as you only get 10 frags per 10x seal pull which means you actually need about 60-100 seal pulls to get her to 3 stars and even then it's not 100% that you will get her to 3 stars with 100 seals might take more, on the other hand most of the time her frags are very cheap like 1 frag/0.3 of a whole seal scroll means 28-40 cp/frag which is very cheap compared to getting her from edo seals. at 2 stars you might as well use anything else as she is very squishy.