Having been on the rescuer end of things for a long time, I would like to say that pretty much every Harbor Patrol Boat (USCG Boat, station etc.) in the US has a Radio Direction Finder on board that will home in on your VHF's signal. That's a direct connection between the agency that is on the water plucking you out.

With a cell phone, at least here in CA, if you dial 911 you get the Highway patrol, then they call they local authorities, then they call harbor patrol, then they call the USCG.

I think the VHF is much more efficient, therefore a safer option.