The thing is, there probably is no way to determine what is land and what is sea, unless you demarcate the boundaries (e.g. by storing the co-ordinates of the boundary points). If you're generating the land mass algorithmically, then you could use the same algorithm to obtain the points.
An alternative would be to slice up the map into larger units, and then set the boundaries. You would then have fewer points, and so the task becomes easier to do by hand.
