Why is water blue at all?
The ocean is blue because water absorbs colors in the red part of the light spectrum. Like a filter, this leaves behind colors in the blue part of the light spectrum for us to see. The ocean may also take on green, red, or other hues as light bounces off of floating sediments and particles in the water.
-NOAA’s National Ocean Service
Warm water has naturally less plankton
The blue color of water shows up best against a light background. Warm water has less oxygen than cold water and less plankton.