You need to toy with the map_height.
When you give a higher value to land pixel (like 5-5-5 instead of 1-1-1) the land is "pushed" into the sea, and the same is true with the water pixel if you decrease their value (0-0-245 instead of 0-0-253).
There are great tutorials around; once you get it, it's easy, but very long.

Don't forget to delete the map.rwm file each time.