Here is a link to a great discussion about use of VHF v. a cell phone when there is a boating emergency –
a ‘snip’ from the article –
Even though VHF-FM radios are not required by law to be carried on board a boat, Sens and Pagels [both with the Fifth Coast Guard District] recommend all recreational boaters, even in the smallest vessels, not leave the dock without VHF-FM and use it at the first sign of distress. Calling 911 with a cell phone should not be ruled out in case of an emergency, but both experts agree that using a VHF-FM for distress calls is a surer way to get the help you need, faster. carry those VHFs!