The Coast Guard explains that cellular and VHF systems provide different services. A VHF distress transmission can be heard by nearby vessels and marine responders, while a phone normally connects only the parties to one call.
VHF also carries marine safety broadcasts and uses standardized distress channels. Cellular coverage, battery life and the ability to describe position can all fail.
Carry the phone as an additional tool, protect it from water, and use suitable marine communications for the route.