Why is the sea bluer in the Caribbean?

When photons in sunlight strike water they collide with the atoms that make up H2O molecules and with floating particles or phytoplankton. The impacts send the photons scattering in all directions.

Essentially, the colour of those that are scattered upwards contributes to the hue that we see from above. A range of wavelengths of light is scattered by floating particles, from the longest (ie red) to the shortest (ie blue and violet).
If phytoplankton are abundant in a certain area, the chlorophyll they contain will scatter green light just like it does in land plants, giving water a greenish-turquoise tint. The Caribbean contains relatively few suspended particles or phytoplankton.

Hence, the majority of light scattered to the surface is at short blue wavelengths. This effect is exaggerated when cloudless blue sky is reflected on the ocean.

Answered by Michael Simpson