I was thinking about taking my drone on a boat. Is there a way to set the Mavic to hover to a gps return to home location? (And not land in the water)

Provided your phone has a GPS, you simply swap the home point from the default of "Home Point: Aircraft" to "Home Point: Me".  The latter being where your phone GPS is as it travels with you on the boat. It will then effectively follow you if you hit RTH. Up to you to ensure it does not land in the water though ;-)

You could set a homepoint on land, then go on the boat.

I cannot experiment yet with the DJI Go app, but if you have android, you might spoof your gps location easy and set that location as homepoint in the app then, most apps then will show that location, unless they do some other specific checks you are spoofing.