I always thought you can not go wrong with giving a gift card to their favorite restaurant.
Most boats have cushions on them, if they own their own boat, find an embroidery shop and have a couple of throw pillows embroidered with the name of their boat on the throw pillows.
If you are crafty, you an make an anchor shaped plaque. You an write the last name down the center column. Their first name across the top cross piece, and "Est. ____" with the year they were married along curved bottom part.