DEMONSTRATION VIDEO: http://www.youtube.com/watch?v=xmEjZK5LN4k

Dig a hole (~10 blocks down). In top, close the hole (use grass or rock or wool or what you want).

Place a boat in the roof you created before. Touch it with the character, until it "partially enter into the block". Now, enter in the boat, and look the magic! Try to place water, or to break a block and place water in it. Now, exit from boat, and you will see where the water is really placed