The ideal thing is to prevent such a thing from happening. Be sure to check out your dive operator and go over rescue action plans, motor operation, history of the crew, and what the system is for counting heads. There should be a log to check names in addition to a head count. It is also wise to ensure that the boat has a functioning radio. In addition, you will want to be a good navigator, listen to instructions before starting, and if doing a drift dive start by swimming up current. You should have with you an inflatable sausage, a whistle, a mirror or other reflector, a slate, and a line.
If you are left behind remember your seven steps to survival: recognition, inventory, shelter, water, food, signals, and play. You can also take these measures: inflate your BC; drop your weights; inflate your sausage; flash your reflector; blow your whistle; write the time, location, and speed of current on your slate; if you choose to swim, then swim diagonally into the current towards the shore; stay with others who may also have been left behind and tie everyone together if possible. There will be rescue attempts so you need to make it as easy as possible to find you.