This is the rule that we use:
If you are hit with enough water to create a splotch that is larger than your clenched fist or an unbroken slash, longer than from fingertip to wrist, you are eliminated.
This allows guns with high output to fire a minimal amount of water for a kill (the splotch).
However, smaller beam weapons can also be effective when using this rule, since even an SS-50 can create enough wet area in one slashing motion to result in a kill.
This system however still relies somewhat on Honor, so it might not be the most effective method for Tournaments.
Terrifying, but oddly refreshing.