I would like to point out that it somehow seems odd to me that people leave their vessels to go scuba diving and leave all of their "survival equipment" behind. I'm aware that in some locations, this doesn't usually present a hazard, but for some of the dives we do here (Canadian North Atlantic), you could find yourself on your way to Ireland if you aren't careful.
If I'm diving off-shore, I carry a personal survival kit with me (designed to keep me alive and responsive for 48 hours). This doesn't seem like long, but it should be all that's required for SAR to respond. In any case, that's about all the time I have. A diver wearing a dry suit will not likely survive any longer than 2 days in the local water temperature (30-36 degrees).
My kit includes: sufficient water, energy bars, a personal EPIRB (which transmits Sat location as well as a VHF emergency signal), a laser signal light and a ACR Firefly strobe light. This is sealed in a pressure resistant container. I also carry a 10' surface marker buoy (DSMB). BTW, I'm an ex-Navy diving officer, commercial diver and military land/sea survival instructor.