Why is the water at Nusa Dua and Melasti beach in Bali sometimes murky with rough waves?

At the main part of Melasti beach the waves were strong, but near low tide and further along the beach the water gets much calmer and clearer — the tide makes a big difference.

Over 6 nights at Nusa Dua, conditions were different every single day — from murky waves strong enough to knock you off your feet to perfectly clear water. You can't judge the beach based on just one day, it changes fast.

On some beaches the murky water isn't due to pollution — it's caused by yellow sand stirred up by strong waves. The beach itself stays clean, the water just doesn't look transparent.