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?