This may help
http://www.twcenter.net/forums/showthread.php?t=70584
Under the Patronage of Lord Condormanius
look ceaser this is a known problem !!!! hmmmmmmmmm
i solved it after a hard work and search like this :
y need to have photoshop to edit the (tga) files map_heights)
if y have photoshop then y need to use this color : (R:1 G:1 B:1) and draw on the coasts and y will have a tidy coasts