Nice map so far. I can't wait to get to download it.
I remember that earlier you were discussing how it would be impossible to add water to the map without it covering the whole map. However, a while ago I saw a build (detailed city build) where someone managed to fill a specific area with water (in a pool and in a river going through the middle of the city). As far as I could tell it was normal water, just not covering the whole map at a certain hight. Underneath the build and the water was a subway, but it wasn't underwater. I'm not sure how they did it, or if it would be possible to use it for your map though.