Scroll down quite a ways and you will find swimming sprites. You may need to do a custom sprite to fit your character. If you have trouble with this let me know as I have done a couple sprites for my own game. You will need to add an event I assume though wherever you want the character to enter the water. Make the event be set below the character and on a button press. then in the event have a message like "Do you want to swim?" , then Show a Yes/No Choice and under Yes is where you would need to set a move route on the player (first set through on, since going onto water is normally not allowed), then move into the water and change the graphic to your swimmer. you will an event also in the water for when you want to get out asking pretty much the same thing.
EDIT:
Anyone else have an idea of how to restrict him from going through all objects (like walls), but allow him to free swim. Maybe he should set the water as a passable tile, limit his entry points, and prompt him to enter?
