It looks really nice! I would definitely play on this map.
For the underwater question, I just posted a Deadly Seas mod and am about to post another that I think will successfully remove all underwater stuff, so map-makers won't need to make "no seas" versions.
looks great, tagging to keep up with progress. I'm all for the multiple .map files with one image and I'm always good with new maps. I'm sick of Aran and Cradle.
I like the idea of multiple map files as well... there should be good options. I will also make separate maps for each of the islands alone definitely.