Well if you want to go full water you already have a good amount of the Greninja line for a Greninja Break deck. That deck doesn't run any EX/GX cards normally so if you really wanted to play with those it wouldn't be the deck for you. Do you have a bit of a collection or is this all you have so far or what's your budget to improve the deck? Fortunately Greninja Break is one of the cheaper competitive decks out there. Also, what are you goals for the deck? (Serious tournament play, mess around with friends, play at a weekly league, play online only?) What you're looking to get out of it may affect the recommendations as to either getting top tier cards or just stuff that is good enough for what you're looking for without breaking the bank. Lastly, are you looking to play Standard, Expanded, or both?
Also, be aware that Sun and Moon just came out, but there hasn't been a major tournament with the new cards in them so it's hard to say how the new cards may improve or hurt a Greninja deck.
Ok, that's out of the way time for some actual recommendations, assuming you want to go with Greninja Break and play Standard. I'll try to put a brief explanation of the changes also.
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Definite removal:
-All grass cards (Obvious, since you're going for a water deck)
-All non-Greninja line Pokémon (Greninja is typically a very focused deck on JUST the Greninja line and a maybe a few supporting Pokémon.)
-Fighting Fury Belt and Max Elixir (Greninja is an Evolution based deck. These only helps basics)
Definite additions:
2 or 3 Greninja Break (Your main source of damage, but also the most expensive card in the deck)
A full Greninja line 4-4-4. (Make sure the Frogadier is the one with Water Duplicates as the attack.)
N and Professor Sycamore (For Draw)
VS Seeker (To reuse supporters)
Lysandre (To grab a specific enemy to target or stall with)
Dive Ball (Search)
Super Rod (Resource retrival)
Optional removal:
Shauna, Tierno, Misty's Determination, Great Ball, Potion. (These are outclassed by other trainer cards)
Professors Letter (I personally like 1 sometimes, but I would not run 2)
These optional ones are ok to keep if you're on a budget, but if you can get them there are better ones out there as listed in the definite additions.
Optional additions:
Talonflame STS (For a good starter)
2-2 Octillary (For extra draw)
Max Potion (Your attackers only need 1 energy so discarding it to fully heal is ok)
Fisherman (To retrieve discarded energy)
Evosoda (For another way to search for your line)
Ultra Ball (More search, plus can grab Talonflame out of your deck if you don't start with it so you can discard with a Sycamore or other UB)
Splash Energy (Let's you send the line back to your hand when it gets KO)
Rough Seas, Faded Town, or Silent Lab (The common stadiums in a Greninja deck, most just pick 1 and go with it)
Pokémon Ranger (Helpful in the mirror match to turn off their Shadow Stitching for a turn)
Bursting Ballons (Really helpful in hitting some good damage counts to keep up with the prize trade)
Ace Trainer (You're playing from behind with your setup time in the early/mid game, why not mess them up a little while you're at it?)
Another Wally or two. (If you can turn 1 Wally directly into Water Duplicates then you can get all your frogs out really quickly)
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Your budget or what you have might affect which additions you can incorporate. Plus there's a bit of personal preference, as there are two main versions. One has Talonflame, and the other has Octillary. If you can, I recommend experimenting with what you feel best playing.
If you have more questions feel free to ask, or if you wanted to go with something other than Greninja Break that's cool to. I'm going to stop writing just in case you don't want Greninja. At least this will be here for someone else.
