1)So, from a gameplay point of view, the map is quite good. Regarding the visuals, maybe more tilest variations in the shallow water are could also improve the visuals. On the highground, you could draw some interesting patterns with the various "round field" tile (or whatever it is called in english), that might even be enough without any additional doodads.
Regarding gameplay:
2-Creepcamps and drops and distances are good; the only issue I see are the expanions (see below)
3-For the merccamps, I'm not sure. It might work out as the problematic deepseer creeps take a long time to become available. Whatever you do here, someone will complain. Another option is to go with a laboratory instead.
4-Whatever you do, you should make it possible to either build a farm at both camps (recommended) or at none, but right now one can build a farm in the south but not in the north. Also nightelves would certainly be thankful for a tree nearby to have wisp for easy access.
5-The same also holds for the taverns.
6-The expansion would more clearly "belong" to one player. There are at least three factors that go against fastexpanding: Long distance for militia, easy to reach for enemy, strong creeps. However, at least the enemy needs to scout where you want to expand. As it stands, I would expect any human to veto this map vs undead.
7-There is only one access to the sea giants and turtle creep stops, which therefore can turn into a trap. Here is screen how I would consider redesigning this map to have some more interestingpathing
options, but of course this is also a question of personal preference.

8-Having these 3 creep camps in line with a neutral building in the middle at each side of the map look a bit unnatural. Maybe you can move a few of them inwards; for example the crabs could placed have-way between the spawns and the current position of the taverns, near the ships.