Take it gradually. Walk him by the ocean every day with people surrounding the horse and whispering encouragement. Don't get him wet yet. Then pet him with wet hands where he will first feel the water. and murmuring words of encouragement while he is tied up and you are on the outside of the stall in case he freaks. Have a nearby water bucket ready to dip your hands in the sea water again and again, starting from 1 minute, to 2, to 5, to 15 etc. Generally, grow in the time.
Once he's adjusted to that, put the riding equip. on him and continue to stroke him again, and walk him by the ocean and pet him with wet hands.
Once he's adjusted to that, bring him a little closer to the ocean and stroke the wet hands again, then closer and pet him. Then, hold him at a safe distance from the ocean and walk him through a small puddle you made. Do it again and again until he's okay with that.
Little bit brain-dead, but that's what I can muster.
Also keep in mind, he might'v had a traumatizing experience with water, and considering he's 27, he probably won't live much longer, and training him to tolerate water might not be worth it.