If you keep flying from the Harbor (swim outside of the Dragon Shield... or you can narrowly fly past it) you'll eventually get attacked by ballistas.
Among there you should find a transporter (not to be confused by the Way Point Shrine, which is there too), which has a door next to it.
Enter.
If that description was bad, Lancet probably has a better one at;
http://www.gamefaqs.com/pc/998546-divinity-ii-the-dragon-knight-saga/faqs/62264
