How do scuba divers keep from floating to the surface of the water?

Answer 1

The Marines have a special kind of Scuba system (called a re-breather) that recycles the air for them, which prevents the bubbles from being seen from enemy sights. In recreational diving however, we only have a semi-closed re-breather, which partially recycles the air. It does not completely eliminate the bubbles, but it reduces them.

Answer 2

In one word... weights. Although it is all really about buoyancy.

Basically, people wear weights to make them sink. Then by adding small amounts of air to your BCD (Buoyancy Control Device) and controlling your breathing, you can 'cancel out' the effect of the weights when you get to the bottom to make you neutrally buoyant (effectively like being weightless).

A little more in depth;

Buoyancy is determined by an object's volume and weight. If the weight is more that the weight of the volume of water that it displaces the object is negatively buoyant (for example the weights that a diver uses). a SCUBA Diver uses this to get to the bottom, then when there adds air to the BCD. The air weighs less than the volume of water it displaces and is therefore positively buoyant.

Balancing the negative buoyancy of the weights with the positive buoyancy of the air should make you neutrally buoyant as the weight of the volume of water displaced is equal to the weight of the diver + equipment.