how comfortable with flying in manual and attitude mode are you?
simple solution: lift off in ATTI and follow the boat lol
home location will be the gps coordinates of where you turned on and initialized the mavic. so on a moving boat that home location will be open water...that would be bad if you activated failsafe...
complex solution would be to use Litchifly flight software, which has a feature called dynamic home. it sets the home location where the controller is. moving controller; moving home point https://flylitchi.com
compatible with the mavics, phantoms and inspires.