What does buoyancy have to do with submarines?

Buoyancy is extremely important in submarines. To sink or dive, they must achieve a very delicate 'negative' buoyancy. It could be very dangerous to sink too fast. Water is pumped into the submarines ballast tanks to make them sink, then later is blown out by compressed air to give them 'positive' buoyancy and make them rise. - While cruising underneath the surface it's very important to achieve a state of 'neutral' buoyancy allowing the sub to rise or fall according to the movements of it's 'planes' ( a wing-like device similar to aircraft 'elevators') . The 'planesman 'on any submarine has a very important job of keeping it at assigned depth.

To complicate this, the sea is not all the same - under the surface there are currents flowing almost like rivers, and 'saline' layers of different densities of water. All of these can change the depth of a neutrally balanced boat.