That actually seems to be the more reasonable case. Hey, it does push people back so that's pretty good.
... No, it isn't. Water Shuriken:
a) Has no KO potential
b) Is a multi-hitting move, which means it can be SDI'd out of
c) Carrying people away prevents follow-ups/set-ups. You want a projectile with hitstun to keep them close so you can follow up with something.
That said ...
a) It still seems like it can carry people, and MAY have negative SDI multipliers to make it more effective. As such, using it to carry people offstage and put them in a bad i.e. edge-guard situation seems useful.
b) Greninja seems to have insane speed and mobility so he can probably get a follow-up off Water Shuriken anyway, depending on the distance thrown and amount of hitstun suffered as well as if the move can or cannot be easily SDI'd.
c) There is a chance that Water Shuriken does not disappear when it hits a shield and all of its multiple hits strike, which would give it good shield-stun and probably let Greninja get a grab out of it easy.
It's probably not a bad projectile, but what we know of it for now has me skeptical. I just wanted to convey that the "carry away" property is not inherently useful