I'd say NO to skewering for Water Main. That mystery is very situational.
Before you can even use it you're probably half dead before round 2 even starts.
I'd rather use heal or shark instead.
To make skewering efficient that Water Main has to take front line to make best initiative before Roshi takes you down with ignite.

Spam healing has the least chance to win u won't heal fast enough against that team if he focus only single target with aoe.