Hey, I'm not sure if this'll help, but, I've had to do the same sort of thing in the past, while some sounds we got with the hydrophone turned out well, alot of those effects we needed were achieved by processing sounds recorded above water.
Some things we tried.
-Different sizes of hoses for bubbles and bursts.
-Balloons filled with air(or water), being released underwater
-Use a hose and spray it underwater at different pressure levels.
-Different objects sort of "slicing" through the water(a rake, long sticks, cheese grater, potato masher, nets, etc)
-Low pass filter some wind sounds and mix with rumbles and bubbles to get some underwater ambiences.
-Use a sampler, Load a bunch of sounds of different size streams, rushing water, bubbles, and fizzy sounds, make whooshes by playing with the level with an amplitude envelope, maybe some filter sweeps to give a sense of motion
I read something Rob Nokes said about hydrophones, basically he likes to use hydrophones arranged in some sort of an array to make the sound longer and fuller.