system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: Why do warmer climates have turquoise, clear seas and white beaches? Why is my sea so gross in comparison?
gpt
If you are imagining tropical islands the answer is that the sand is primarily derived from coral atolls/reefs.  The coral has a white skeleton when it perishes/breaks away.  

The reason the water is blue is because these islands are the only land masses in an open ocean.  The open ocean has significantly less nutrients than near continental landmasses and hence there is less plankton / sediment colouring the water green / brown.