Based purely from a GM perspective (and not having searched through FAQs, RAWs, etc.) it seems like a clever use of a power that, in my mind, is not exactly overwhelming to begin with. As such, I would probably allow it provided everything was timed right, etc. As for how much of a boost though, that kind of depends. I think it would be hard to pull off well from a "realistic" standpoint (yeah I know, we're talking about magic and dragons and elves, Oh My!), so I don't think I'd give a huge boost on it. I think the ideal use would be to use it at the top of the person's jump, as using it directly under them right as they are about to jump would mean they are trying to jump off of water, which would be substantially harder imho. But use it to give an extra boost at the end? Probably not too big a deal.
RAW, it seems to me that since the spell allows you to "perform a bull rush" and bull rush "pushes opponents away from you," you can't pick the direction. But I would desperately wish to be wrong.
It is unclear with regards to Hydraulic Push. It is a close spell so its point of origin of the effect has to be with in the "close" range. Therfore you should be able to "push" in any direction.
Hydraulic Torrent is a 60ft line which does imply it originate from the caster. So creatures will be pushed away from the caster.