If you are in danger
Call the coastguard on VHF channel 16. They handle emergencies at sea and will send help.
If you are safe
Contact the charter company. They know the boat and the area, can tell you what to do next, and can arrange help if the boat needs repairs.
If a breakdown cost you part of your trip, see Do you offer compensation if I wasn't able to use the boat due to mechanical issues? and What if something goes wrong during my rental?
