The real answer is it depends how the diver is exerting force as he stands there and what force he has to overcome to jump forwards.

If he’s using all his driving force through his legs just to stand there then when he jumps up he will go backwards.

If he was casually standing there easily overcoming the push of the air already then he can jump into the pool just as normal.

It’s the same as standing next to a swimming pool with different wind speeds. Small breeze = jump in ok. Hurricane = blown backwards.

That’s assuming he wasn’t trying to jump off the back of the vehicle…