6th Gen Adamant Nature Greninja for In Game?

Just restarted Y version and picked Froakie. Got one with Adamant nature. Is Water Suriken any good in this game? will a physical Greninja be alright when I get to the E4? Been a long time since I played this game so forgot a lot of info.

Water Shuriken is better in Gen 7 with the change to special and the slight boost it receives when Greninja is Ash-Greninja. Not really worth using in Gen 6 imo.

Greninja doesn't have a terribly impressive physical movepool without ORAS move tutors (not the you'd be able to make use of them pre-E4 even in ORAS), but like, in-game, so maybe you could make it serviceable in X/Y.....

What about Waterfall then? I know at least Night Slash will be good but she wont learn that until lvl 77. I doubt I'll be that high before the E4 but maybe idk.

Yeah, Waterfall would be your go-to Water STAB. Night Slash you can learn earlier than that via the move relearner in Dendemille Town, as it's also listed as a level 1 move for Greninja, so you get it that way.