Perhaps you can train him with two bowls of
water, one
salt, and one fresh. If he goes for the
salt water, you tell him no very firmly, and when he drinks the fresh you reward him with a treat. After a while, take him to the ocean, and if he drinks it, you tell him no, but if he doesn't, you give him a treat and be nice to him. Labs are very smart, and I bet he will pick up on this quickly. Good luck.
Also, be sure you always have fresh water for him to drink on the
boat.
Curious, where does he pee?