No Mystic Water
Right off the bat that's my answer. That said, consider the following:
- Greninja has very few good Physical Water type moves, so you're probably better off using some variant of Protean Greninja which relies more on U-Turn/ Acrobatics to deal damage than Waterfall or Dive
- Mystic Water boosted Waterfall/ Dive is still relatively weak compared to (in most cases) a strong neutral move like no item Acrobatics specially if you're running Protean.
- A Choice Band/Scarf will do wonders compared to Mystic Water (Scarf for revenge killing Mega Alakazam and Scarf users)
- No item Greninja could in some cases be a niche switch in to a predicted Knock Off as you resist it, allowing you to maintain some offensive pressure.
- A good item would be Grassy or Psychic or Electric Seed, which is immediately consumed of you're in the respective terrain, which is easily set up by your own Pokemon, or is common enough to take advantage if the opponent does (but is risky as they might have the wrong Tapu). The seed is immediately consumed, allowing you to fire off strong Acrobatics right away, and also boosts your shoddy defenses (though it's still a glass cannon capable of being OHKOed)
All in all, a big no-no to Mystic Water, it's generally a sub-par competitive item
Edit: An earlier version of this answer suggested a Waterium-Z under the mistaken assumption that the Crystal is "consumed" upon activation of the Z move, thus boosting the power of Acrobatics. A quick test on Showdown proved this assumption wrong and the suggestion has been replaced.