Gawain never rides on a ship on the ocean that takes him towards the grail. However, on his adventure, Gawain comes to a castle full of maidens. To get to the castle, he needs the help of a boatman:

While Gawain was still thinking about the maiden, he saw a punt heading towards him from the direction of the castle, piloted by a boatman; when he reached the shore, the boatman said: "Sir, I bring you greetings from those damsels, and they also urge you not to keep what belongs to me; return ti to me, if you please."
Gawain learns that the horse that he is on, which he won from another knight, is the the boatman's entitlement. Gawain, in need of a horse himself, strikes an agreement with the boatman, who promises to keep the horse in trust, for Gawain while Gawain is in the castle. He then brings Gawain across.