I havent read the Delta FOM in a while but most major airlines and some regionals like Endeavor and ExpressJet (ASA side) have the over water exemption granted by the FAA to save on the costs of maintaining and training for life rafts on a plane. As Saabowski mentioned, it allows carriers to go between 100-160 nm offsore depending on the geographic location.
The WATRS airspace that carriers use to get to Florida and the Caribbean over the Atlantic require specific navigation performance. Company policy and MEL
limitations further can limit your ability to operate WATRS airspace.
For example, many aircraft at an airline have dual FMS/GPS
installed but certain operations require both to be operative. Company policy might also require both to operative. Therefore, having a single FMS or GPS
deferred could mean the flight would need to use over land routes instead of the over water routing.
When you are over water far enough away from the shoreline, you are out of the range of the land based navaids. Thus a plane needs a way to navigate being too far away from the service range of the VOR/NBDs on land. GPS
navigation and IRS systems are the primary means today of navigating outside of the service volumes of land based navaids.
This means airlines want and the FAA requires through the MEL
book and the over water exemption terms a way to have multiple working backup systems where navigation is not predicated on the use of land based navaids.
If an airline had a choice to spend hours trying to fix a GPS
system or to fly over land and burn maybe a little more gas, it will choose almost every time to operate the flight on time regardless of the increased burn enroute. A plane sitting a gate isnt making money.
[Edited 2013-10-25 08:19:21]