I thought most of the rivers in Vietnam were Brown, the new map Cam Lao has lovly clear waters every where, just not right......
Link https://www.navyhistory.org/2012/01/brown-water-navy-in-vietnam/ https://brownwater-navy.com/vietnam/BoatsAlpha.htm
I heard the devs say that they wanted the rivers to be brown, and the ocean to be blue, but you can't do that, so the compromise was to make all the water green.
As stburr91 said, Arma doesn't support different color waters so a compromise for the water colour had to be made.
But the waters around Vietnam were also brown and muddy, I am sure I saw photos of this also.... Would have been better with less of an Ocean more map