Honestly at first I was a little doubtful of the concept of 'undersea', but after looking at the images and info, it looks great!
As some have already mentioned, it does need some additional touches to better the experience for players.
As an FFA frequent, I think that open areas are definitely a plus. However, I personally don't agree too much with the many structures around the centre area, maybe I haven't seen them completely but too many structures around the map might make it feel cluttered.
The many overlays around the map are very aesthetically appealing, but might also cause certain problems to arise. Oil rig is a good example as most of us cannot even see clearly at the bottom floor, which logically should be the most convenient area for PvP as it is very flat.
Overall though, great job on your efforts of building this map, I hope to see it in use one day!
I don't know much about building maps, and the above is solely my humble opinion.
-Winkeo