As Heather and Brian pointed out, the nearest Coast Guard Air Station is in New Orleans, roughly 160 nm west. After that, you have CGAS Clearwater, roughly 270 nm to the ESE, but that is also in a different district (District 7). Therefore, helo evac is not going to be a viable option from the ORISKANY site unless for some miraculous reason a unit is nearby by shear happenstance. You have SAR units in both Pensacola and Destin, but they have no air units stationed there.
Lifesaving-related activities is one of the primary missions of the USCG, but they are a heavily-tasked entity, even moreso now since they have been folded into Homeland Security. They are definitely a tremendous asset to have out there should the fit hit the shan, but if you are expecting them to operate like Domino's Pizza and be there in 30 minutes or less, you need a dose of reality.
To the best of your ability, prepare and plan on handling dive-related emergencies with the resources you have on hand (or had the foresight to bring with you) when offshore. If and when the USCG show up, that's just gravy.
It is interesting to note that when Escambia County made their pitch to the state of Florida (FWC) during consideration of where to deploy the carrier (NW or SE Florida) they presented "data" demonstrating that response time was not a significant factor in dive-related emergencies
