"TRAILS"

That's the hard way. Mark the marina as a favorite and then you can turn the unit off. When ready to return, turn the unit back on, let it acquire satellites and then tell it to navigate - off road - back to the marina.

It will probably try to take you in a straight line, but at least you'll be headed in the right direction. The other way is by setting the unit to show your track, but it will not retrace your outbound track.

To turn on the trip log ... Tools - Map - Trip Log - Show. Note that the trip log only saves 10000 points (or bread crumbs), then overlays itelf. There is also hidden track logging feature in the nuvi that will save multiple track logs. There is an extensive discussion about the hidden track logs in the gpspassion forum. Just do a Google search and you will find it

The trails function is great for boating. Obviously you need a "real" chart as well, in order to find navigable channels and which place you go and cannot go.
1. Before you set off on your trip, mark your start point using set "present location" as a Favourite. Set the trails function and and record the actual route taken to wherever you are.
2. To return, set the GPS to goto the Favourite, you may also have to set the unit to "Off Road"
3. You now have a straight line home, however, you probably you cannot go straight.
4. Enter vias, as necessary, to follow your original route.
5. You can also enter any locations that you may which to return to. Enter as if you were on land.
6. The GPS neither knows, nor cares that you are on water. It performs as if you were on a wide open plain. Just remember that it also doesn't know, nor care about any hazards to the boat. You do need a chart to tell you those.