The palm trees already grow in a tree crop plot. Perhaps there could be fruit trees? Oranges, apples, lemons... (I should note that citrus fruits are logical on the ocean, as vitamin C is needed to prevent scurvy. Yes, scurvy is a real disease and not just an insult thrown at low-ranking sailors.) Tropical fruits would be the main reason to have more trees. Other than that, I can't think of much else. Maybe some trees produce a sticky resin for water-proofing things? Resin could be useful for making boats.