In an emergency please use your VHF radio first...
The Coast Guard and local Harbor Patrol or Bay Watch is monitoring channel 16 at all times.
You may be able to use your 911 function on your phone in an emergency ...but...as I've found out as a weigh boat on a few tournaments...you can be very wrong as to your actual location when transmitting that information to the operator. "I'm about 3/4 mile off the beach in front of the white condos"...in reality... he was 1.5 miles off the beach and there were a ton of white condos on the shore...LOL
When you broadcast from a VHF radio the Coast Guard or Harbor Patrol can use their Radio Directional Finder to get your approximate distance and location...and facilitate your rescue....
From Boat US..."In order of priority,
MAYDAY is the internationally recognized distress call that is used as preface in VHF radio transmissions only in situations in which there is an immediate danger of loss of life or the vessel itself. This includes when a boat is sinking, there’s a fire in the engine room, or someone on board is unconscious or experiencing a serious injury or illness.
PAN-PAN is the international urgency signal that is used as a preface to a VHF transmission when the safety of a person or the boat is in serious jeopardy but no immediate danger exists, but it could escalate into a mayday situation. For example, pan-pan is used in situations in which the boat has a slow leak or the engines are disabled and the boat is drifting toward a rocky shore.
SÉCURITÉ is a safety signal used as a preface to announce a navigation safety message. This may be an approaching storm, a navigation light failure, a submerged log in a harbor entrance or military gunnery practice in the area.