This bug was last reported in the 1.13 pre-release, was not acknowledged and is still unassigned. It's most likely that Mojang is not aware of this. As mentioned previously, please vote on the tracker to help it get attention.
The Meaning of Life, the Universe, and Everything.
I would say, based on the fact that the water assets provided by the game for 1.13 are grayscale images, that the blue color for water is a Works As Intended feature. Water getting its color from the game according to biome just brings it in line with other, similar assets like leaves and grass. You'd be better of lobbying to change it back, if anything, since it's almost certainly not a bug.