My "road only"
Garmin has a hiking mode that will give a latitude/longitude position which can then be plotted on a
marine chart. Also would show your position on the
water but of course without
depth, markers, etc but still somewhat useful for navigation.
When it was really handy was when we were trying to find the
dock for a
water front restaurant in a strange town. Programmed in the address and lead us right there.
So, not the perfect solution but I would take it along as a spare if it had a lat/lon mode.