If so, what setting did you change to do it.

You have to pay attention while using the RTH (return to home) function, because the place were you are standing (in your case, ship or boat) is moving and therefor the coordinates are anymore the same.

So, pay attention in case you lose signal and the Mavic initialises automatically the RTH function. I wouldn't suggest you anyway to use it on a boat or ship..at least as long as it is moving.

Which setting do you need to change? Have done it with my P3 and have launched/retrieved the Mavic by hand the same way I would the P3.

No settings need to be changed. I would probably hand launch/catch to mitigate the effects of the boat bobbing up/down. Also you cannot use RTH because even if you reset your home point at the end of your flight, the boat will still be drifting and the RTH point won't be accurate for long.