All sorts of things:
They only appear to have a couple of buckets of water going down that (maybe a couple more in the background) – nowhere near enough to get him to slide on that at the speed he needs, particularly given the up bit of the ramp will be totally dry.
Listen to the "audio" – still accelerating as he goes up the ramp.
The pool doesn't move, despite the large horizontal momentum.
Vertical splash.
Most conclusively of all – the speed he'd be going to make that gap, there's no way he'd be standing up from that landing.
The middle bit would appear to be CGI (maybe a dummy with CGI, but all CGI seems more likely, as I still don't see how they'd get the dummy to fly that far from that ramp).