Can't help you on the boat front, but the best gravadlax I've ever had was mackerel. With a divine sauce which, if I remember rightly, was creme fraiche, mustard, lots of dill or fennel, S & P, and a squeeze of lemon or lime. Superb on home made Irish Soda Bread, sliced very finely. Mmmmm. Need to find a boat on the Firth of Forth.