http://youtu.be/akxqOJ1kzKA (also contains another bug I am also reporting in another issue)

place a water bucket anywhere

hold the now empty bucket in the offhand slot, and the empty bottle in the main hand.

Put your cursor just above the water, then quickly flick your camera down, right click, and flick it back up as fast as possible.

Often, but not all the time, both will fill and the water source will go into the bucket.