Each has their uses. Sakura can be crucial and amazing in PVE events (although if you're water, maybe not for you). She has better forms, so I would not recommend farming her stars.

Karin is a more offensive healer, as she can poison and high combo chase to ensure poison stays active even without her mystery (which sometimes you may prefer over Kabuto or Shizune). She is, however, my least favorite healer so I would not recommend farming her up.

Tsunade is probably the best easy-to-obtain healer, as she can cause Chaos and heal herself for an absurd amount while still healing her teammates. Especially in Ranked Battle, where you can't focus her down, I've lost purely because Tsunade would get low and instantly heal to full AND then cause Chaos off her critical attacks. However, I also would not recommend farming for her fragments either as she has a Hokage form that seems much better.

Especially as a Water main, I don't think you really need another healer. For me, running Earth or Water is for you to take up the "tanking" or "healing" roles, respectively, with your main character and allow you to add in more damage/control/etc. If you really like them, then feel free to get them to I'd say 4-star max (Because from ScionStorm's video it looks like we'll need all the stars we can get for the future shop system). Even so, I'd rather farm better ninjas.

Yeah, he is more easy to improve and more useful tbh.