After taking off, you can manually reset the home point in the following section of DJI GO:
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The icon on the left will set the home point to the Mavic's current location. The icon on the right will set the home point to your mobile device's current location. The latter can only be used if your mobile device has a GPS receiver. Since this is a manual process, you'll need to keep resetting the home point each time you move your boat to a new location.
Another option would be to tell the Mavic to hover in place at its current location if the remote controller signal is lost. You could then steer your boat toward the Mavic until the remote controller reconnects. That can be configured here:
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