Dancing Hoskuld
Deity
Islands can be similar to the reefs in the way they treat water units, but to improve them with work boats, and cannot be landed on by land units.
It just kind of occured to me, when I was looking at the map.
The natural harbours, essentially treated like coast, with the port upgrade can be treated like a canal, which would make more sense than using a 'fort', and can get improved in cohesion with the harbour buildings.
I hadn't really thought it all through yet, I just wanted to see if the idea would be acceptable before I fleshed it out.
The island idea is doable just need to figure out some graphics. May even be able to use a version of the goody island graphic, shrunk. I'll add it to my list for looking at.