Submarines
Radio Controlled Submarines use two different methods to control their diving. Dynamic remote control submarines are more simple affordable, they use their forward motion and dive planes to dive and travel underwater.
Static RC submarines are more expensive and use complex controls to remain submerged. The static diving submarines take on water to the point that they will easily dive under the water with a little bit of help from the control surfaces (dive planes). To surface, water is forced out of the ballast tank either by a pump or by compressed air.