Boat leaves from the center dock outside of the Magic Kingdom. They should run about every 20 minutes, but that can change due to pilot experience, weather or other things.
You want the boat that has a green or a red and green flag in the front. If it's red and green you're looking at an extra 10 minutes to the trip.
Plan on it taking a minimum of 30 to 45 minutes to get there from when you walk out of the Magic Kingdom gates.