They're a continuation of the continental shelf. The apparent light color of the water is a reflection of the light-colored sand beneath the shallow waters.
It's like looking at the bottom of a swimming pool from the diving platform. (I've done it, and then went back down the ladder; was too young according to base rules to use the platform, and just as happy not to!) When looking at the water from the side of the pool, it looked pretty dark blue, but when I went up onto the platform (10 meters above the surface of the water), the water looked much lighter, like the walls and bottom of the pool.
Sunset Zone 17, USDA Zone 9