Deck time, cockpit time, saloon time, kitchen time, bedroom time, etc!
Imaginative play, arts & craft projects, card games and board games, legos and magnatiles, science experiments, helping in the kitchen, hammock, activity books, story books, spontaneous made up stories, audio books, sharing memories, swimming off the back, kayak and dinghy rides, playdates, and lots of free, unstructured time for him to come up with whatever comes up!