I'm with AK, usually tip about 15%. General suggestion, 10-20% based upon service provided. I'm usually on the Rooster, and with that crew my tip averages a bit over 15%. Some other boats and experience on a charter, well it can be 10%.
here is a thought, if your new on the boat and find a knowledgably deckhand, talk with him and ask for assistance on the trip and give him a partial tip prior. If satisfied with service, tip 15 percent or more at end of trip