I don't think the game likes there being only one water province...
Gals and guys, how do you go making colour maps?
To start with, I'd take your terrain file as a base. Texturing can be done using cutout patterns from the base game colormap.dds files, along with blurring and smudging (or using different cutouts) to reduce tiling. When it comes to saving, just export as a dds file and make sure to use DXT5 compression.