Where do you get the boat cannon on Legend of Zelda the wind waker?

After you get the Forest Rail Map from the Tower of Spirits, you'll have to head past Castle Town to visit the next area. On the way you'll be mauled by some enemies and have to make an emergency pit stop before you get blown up (which will happen if you don't stop at the Castle Town). While there you'll receive a letter from Alfonso talking about a 'surprise'. He's in the room you woke up in after the incident with Cole and Byrne - in the North West corner of the first floor of Hyrule Castle. Grab him and drive all the way back to Aboda Village, making good use of your whistle to frighten off enemies. Talk to Niko to procure the Stamp Book and head back to the station. Alfonso has a little 'surprise' for you....