The Mayday call is to be reserved for only the most serious situations on the water, those in which there is extreme and imminent danger to life and/or property: if someone is dying or will die without immediate assistance, for instance, or if your boat is in flames or is sinking fast and your are on it.

Mayday should be transmitted via Channel 16 VHF or 2182 kHz MF/SSB.
Try to get as much information across in as little time as possible if you must call Mayday.
The Coast Guard protocol calls for beginning the transmission with "Mayday" repeated three times. Next, call out the name of your boat.

Follow up by stating the position of your boat. If you have GPS, relate the latitudinal and longitudinal coordinates. If not, you can use a distance and magnetic or true bearing from a navigational landmark, or you can even provide the name of your departure point, time elapsed since your departure and the speed at which you're traveling–anything to help rescuers pinpoint your location.

State the nature of your problem and the type of help you need. State what channel you're listening on and sign off with the name of your boat.