Thats because reserve seal isnt a win condition for water vs lightning. The only possible way reserve seal would fully be maximized is if you have a frontline water main you expect to tank hits. Also, unlike sailor sakura who heals and does an AoE splash standard, you would have to sacrifice standard heal to use the attack boost (unless you really want to just boost chase damage).

With the limited niche use of reserve seal, the alternative talents in that line are just way too good to pass up while you run a lineup with a more solid tank.
Addon:
RoW typically wont succeed vs immortal teams, since they sacrifice offence for defence, and the water doesnt care what defence the opponent is running, she's just DoTing you down with poison. Plus RoW only lasts 2 rounds and in that time you can chip away at the 4th ninja thats not immune.

(I'm assuming that since you're considering reserve seal your lineup is more defensive than offensive)
This post was last edited by Slacker~ at 2017-5-30 09:16

Against death hidan, as long as you survive the R1 + R2 first mystery burst, you're golden, his lineup wont be able to win any more once the double heals come out in r2, so adjust your ninjas accordingly to counter this. His sailor isnt going to do anything, you're sailor applies poison at every turn. Identify how you're losing (whether its because of r1 tag, r1 burst, or r2 burst) and see what ninjas you c*e to mitigate this (eg: kabuto clones to soak up tag, gakido for r1 clear, etc).

Reserve seal definately should not be used vs death hidan. Go healing tips.