This is an improved and much more realistic sea water shader version for the game. Just extract the .zip inside your Stronghold 3 game folder and replace everything.
Enjoy! :)
If you are interested, please follow the discussion about an overall environmental improvement of Stronghold 3, here
Note: If you don't want to lose the original shader textures (normal map, foam), for some reason, make sure you back them up, before replacing them.
Excellent Stuff!