The water in Earth's oceans moves from west to east as a result of the planet's rotation. This movement creates a phenomenon known as upwelling along certain coasts, wherein the warmer surface waters of the ocean move out to sea and are replaced by deeper, colder, sediment-rich waters
By contrast, the waters off certain tropical island don't experience this kind of upwelling. Reefs and other physical structures off their coasts act as barriers, breaking the force of the moving water and dispersing its energy. So by the time waves reach the coast, they're too calm to generate the sort of water-churning patterns that stir up the waters.
But it's not just only tropical island that have crystal-clear oceans, and calm waters aren't the only factor at work here. Certain bodies of water, like those off the coasts of the Philippines for example, may have lots of waves and still remain clear. In these areas, the water's clarity has more to do with what's in the water than what's around it.
It's about what's in the water. Somewhat counterintuitively, cloudy, greyish-looking water is often richer in nutrients than clear, sparkling water. That's because that murky water is typically home to more living organisms, including phytoplankton (algae) and zooplankton (jellyfish and other ocean-wandering animals), which in turn make the water appear cloudy.
Sediments play a role in this, too. Along certain coasts, finer particles like sand and silt can add to the water's murkiness since they're easily agitated and stay afloat for long periods. On the other hand, the sediments off clearer coasts may be heavier and coarser. Instead of fine sand, these sediments are often made up of things like pieces of shells and chunks of dead coral, which are often tougher to stir up.
So there it is - some beaches may be murkier-looking than others, but it doesn't necessarily mean they're dirtier or any less worthy of enjoying!
--
Summing it up
• The water in Earth's oceans moves from west to east. This movement creates a phenomenon known as upwelling along certain coasts
• The warmer surface waters of the ocean move out to sea and are replaced by deeper, colder, sediment-rich waters. This has a side effect of turning some waters brownish-grey
• Reefs and other physical structures off islands coasts act as barriers, breaking the force of the moving water and dispersing its energy. By the time waves reach the coast, they're too calm. Hence bluer waters
• Along certain coasts, finer particles like sand and silt can add to the water's murkiness
