As far as I know, Cowana, there are two types of RC sub:

Those with ballast tank(s) (static divers) and those without (dynamic divers). Dynamic divers tend to use propellers to push them below the surface; more elaborate dynamic divers may also have moveable fins.

A lot of RC sub enthusiasts start with the cheaper dynamic divers. Ready-to-runs (or almost RTR) may range from $5 for a tiny thing to $150 for a possibly-overpriced Nikko Seawolf. Kits get more elaborate and higher-priced. I've had fun with Wal-Mart's $20 dynamic, fixed-fin RTR subs available in the US. I haven't tried static divers: Entry-level is $500 - $1,000.

If you've ever done what my two-year-old does--play with water containers in the tub--you may have noticed that, if you fill a closed container to a certain level, it reaches a point where--released at depth--it's not in a hurry to go up or down: It neither pops up like a cork nor plummets like a rock, rather it almost hovers. If you give it a little tap, it goes a long way. That's near the point called "neutral buoyancy" (negative--sinking; positive--rising). Dynamic operators keep their subs' weight adjusted around that point, often using small weights and Styrofoam. That's why we don't need ballast tanks, just a little bit of prop at the right time.

In this photo of a dynamic-type sub, you can see a couple of ports for the thrusters: The one in front of the conning tower is for the up/down thruster; there's another port like that lined up with it on the bottom, so the water flows completely through. The port to the rear of the sub is for the left/right thruster. Also, the sub has a forward/reverse thruster in the tail out of the picture. The control pad has two buttons for each thruster.

You'll notice the sub looks dusty. That's silt from the bottom of a pond: I ran my sub with weak batteries in a pond one day, and a weed may have wrapped around the propeller--and I found the sub two months later when the pond had dried. Sub recovery is simple in pools, where you can always see your sub--but for some of us, there's intrigue in finding a new "ocean" now and then!