Hiya!
Open up the Env/EnvColors_Pac.dat file and find every data class called "WaterLightColor" or "WaterDarkColor" in there - there're 42(?) of them altogether residing under the WeatherTypes->XXX->Color entries. Play around with the variables named "a" and see what happens if anything

Now I'm not sure of this, but I _think_ this "a" is an alpha channel (i.e. color opacity) control variable...