To unlock the You're on a boat! trophy in Borderlands: Game of the Year Edition, you will need to discover the boat in Treacher's Landing.
You will discover Treacher's Landing as part of the story in the Hair of the Dog mission that you receive from Crazy Earl.
Travel to the far east side of the map, and you'll see a few signs with arrows pointing to the left. Instead of following the arrows, go right and follow the path down to the beach and past the rocks and you'll see the boat docked there. Climb on board and the trophy will unlock.
You can see a video guide of the location below (credit to Double) on YouTube):