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.