W.H.
Hey L.. Do you have access to a larger boat? {like a 'party boat'}, that he could ride on? Not sure what your boat is (assuming small row boat or a canoe). Can he express his fear? Is it the feel of the boat on the water? that water will get in the boat and sink it? that the boat will tip over? Does walking out on a wharf bother him? Will he swim in a pond or lake, beyond where he can stand easily, in say waist/hip deep water? Not sure if any of these things will help, or help you get some answers, but hope it's a start!