Geez, Tom, I don't understand. Whether in a sail boat, kayak, walking or whatever, a heading is one of the most common pieces of outdoor GPS data. Every outdoor GPS I've ever owned provided a heading, and that is going back to 1993. I'm confused by your statement.
Do you need a compass on your sail boat? I put an analog compass in my "emergency" bag years ago. It is still there, but I never use it.
I don't consider dead batteries a "GPS failure." I do try to have batteries handy so they can be changed on the water.
Also, the only GPS that ever failed me on the water was my first one which I bought about 1993. Well, I did have a Magellan fail on land in Alaska, because of a combination of user/design errors which resulted in saltwater getting into its electronics. I had a backup--another GPS. I've never had a Garmin fail on water--although, because in the early years I did not protect them from seawater when kayaking, they failed to start at some point—usually at home. Today, our current 3 Garmins are 6-7 yrs old, have generally been kept in a dry bag of some sort, and are still going strong. They have never failed. I'm only slightly embarrassed to admit that I rely totally on my GPS when doing any trips, including 7-8 day kayak or sailing trips in Florida. I seldom take maps on any of my Florida trips. I do carefully lay out routes on computer digital maps, and upload them to our GPS’ (and other members of the group if they have Garmin GPS.) After that I rely on those routes to get me to my destination. If I am in new territory (like Yellowstone last summer), I carry maps, but still rely on routes I have put on my GPS. Years ago, I loved to stare at maps and dream of trips while at home. I don't do that anymore. Today, I stare at Google Earth and dream of trips, but I use my GPS to do trips.
It is my experience that modern, outdoor GPS units are very robust and reliable. They seem to last indefinitely if kept free from saltwater and, to a lesser degree, fresh water.
Also, I am a strong believer in learning how to use your outdoor GPS so that you can rely on it. They are great aids. Use it every time you are on your boat, when you go on a hike, when you walk around your neighborhood. Use it so much, it becomes second nature and far easier than a compass. A GPS provides much, much more info than a compass.
I sail: S FL, Key Largo, Everglades to Cape Romano, Ft Desoto, Cedar Key
"Any intelligent fool can make things bigger and more complex ... It takes a touch of genius and a lot of courage to move in the opposite direction." A. Einstein