Like you I have used SEMS on dash, vinyl door and trim panels and auto carpet on collector cars with good results. One of our club members tried it on a headrest, it was fine for a short while but didn't stand up to the wear. I've have not seen a marine version, not to say it doesn't exist.
What do the directions say about use? Maybe the seat back may be a good place to start.
I just bought a bomb can of black the other day, $11 and change. I suppose the marine version is $25 dollars a can.