I work at a Coast Guard Radio station in Canada and I would highly recommend a handheld VHF radio over a cell phone. Not only are they more reliable for contacting the coast guard (via channel 16 or DSC distress alert) and the dive boat, but many are waterproof and could actually survive being used on the surface in rough conditions.
Also, both US and Canadian Coast Guards (as well as most others around the world) continuously monitor channel 16 and should have very nearly complete coverage within 30 miles of the coast. Cell phone coverage is not that reliable.
Inland it may be different but for ocean diving (or Great Lakes) the radio/GPS listed above looks like it would be a great choice for this type of situation. As with anything, ensure you are very comfortable with the operation of whatever safety equipment you end up choosing, in case you ever actually need to use it.
Hope that helps a little.
~S