But if you're using Greninja for a water type alone...erm, I don't mean to sound like a richard or anything so please don't take it that way, but you're probably building your team wrong.
I'll lay out my team and their roles
(I don't tend to battle competitively online)
Bisharp- Glass cannon, common crits (high speed, wide lens, all increased-crit moves)
Greninja- Water guy.
Venusaur- Grass guy
Infernape- Fast fire guy, has thunderpunch and brick break along with some other fire stuff.
Gardevoir- Cleanup and destroyer. High special attack, moves like moonblast and shadow ball.
Hawlucha- General rough and tumble guy. Fighting and flying moves.