You'll need a ship sprite, and the water tile your using need to be either a) An autotile with passability set to a square, or b) A normal tile with passability set to a circle with the land around the water set to an x.
Make an event to board the boat, have it transfer the player to the top of the water and turn their sprite to the ship sprite. That should let you make a boat that goes through water on XP.
-WCW