HoMM VII Haven Units – Haven faction units from the seventh installment of Heroes of Might and Magic.

It is used to darken the water through a normal map; I don't know exactly. Here are the normal map textures from the closed beta version that psatkha provided. Look at how the water looks with them.

I took a quick look. I didn't notice anything special in the files themselves. One texture is indeed a normal map. Mechanically, it seems to be displayed the same way as the water normal map from the original game, but because the texture from the beta is more angular, it's more noticeable that it is a normal map.
I think you should look for the reason not in the texture files, but in the material files.
I suggest you conduct a test: replace the water normal map with a normal map from Heroes 6, just to see how the game will display it. (I can't do this myself right now because I don't have the DDS plugins installed). Then you can try applying the water material to creature models and, little by little, using the method of elimination, figure out which line and in which file is responsible for displaying the normal map and whether it is possible to transfer this function to the creature model.

Added 2 minutes later

horse cutscene falling down

that feeling when, in the middle of the "revolution," the horse suddenly falls down))