I dont know about the big map size (For a map like water wars, I reccomend a small map size to stop people getting bored cuz they cant find people and so they have to think fast) , but for the so called "islands" , you can cover them with pylon aura sprites to give it that underwater look. But I could be totally wrong, if you have started on the map and it kinda shows what the map is supposed to be like, post some screenshots so people can know what your map really is.
