The previously linked thread is a little out of date now; the way I would recommend to create custom sea/sky is by using the same method as is done in the EA worlds.
The previously linked thread is describing how to modify the basegame files, which use RGB values, whereas EA use colour gradients (ramps) in their custom worlds.
This thread is describing how to obtain and edit those files, and this method has been used successfully in many custom worlds.
http://nene.modthesims.info/showthread.php?t=459198
Please feel free to post there if you have any input or questions.