Number one rule of Scuba diving is never ever hold your breath the pressure changes when you descend and ascend could damage your lungs. When ascending a diver does not need to just exhale they breathe normally unless they are using an emergency ascend in which case, depending on which emergency ascent they are using and for what reason, ie out of air and so on, will exhale on the way up to again ensure the pressure changes will not cause their lungs from over expansion and bursting
I hope this helps
As the diver ascends to the surface the external pressure decreases. If the air in the lungs were not exhaled, the volume in the thorax would expand, and the diver's lungs would explode.
The air in your lungs expands in volume as you get closer to the surface of any water you swim in. This is because the air was compressed when deeper due to the pressure of the water around you. This is why when scuba divers always exhale while ascending to avoid rupturing their lungs as the air expands.
Technical scuba divers do, but recreational divers do not.
By Ditch I assume you mean discard or Jettison. obviously lead ballast which can get in the way!
The collective noun is a bubble of scuba divers.
breath without life support. :)
Many people use a SCUBA suit for diving. Sport divers, police divers and some Navy recovery divers.