Thought about this a while back when messing with my Revell kit.
Basically what i came up with (to do it on the cheap) was to make the model have a slight positive buoyancy.
IN the model i would have two syringes side by side with their plungers connected in tandem to a central threaded spline which is connected to a motor.The point where the needle attaches(Non Plunger end of the syringe)
would have been connected to rubber hoses going to the subs outer hull,(to the water)
Depending on the motors turning direction the plungers would either suck in water(giving the sub a negative buoyancy, as well as increasing the air pressure in the enclosed space), or pushing the water out(+ buoyancy) and neutral air pressure.
I think it would've worked (the more technically minded here can tell me if I'm wrong), but i never tried it.
Steerage n stuff you can get from most hobby shops etc, but i found the ready made diving kit too expensive.
I hope you understand what i was gatting at (it's sometimes easier seeing it in the minds eye than to describe it).
Can always draw a scetch if you'r unsure.
Hope it helps, and it you do try this method, please post as i would love to see if it works.
