Ok - if you look at the coral reefs (aside from clone stamping them directly with photoshop) you can see they have a very bright, nearly white edge, where there might be sand deposited on them. Towards the island you get a gradient going from bright turqoise (a very pale blue-green with a lot of saturation) to a darker blue. In the other direction you see a hint of a narrow gradient going from a darker, inky black-blue to the blue of the open sea. I'd make three layers of stuff and fiddle with their opacity to get a working coral reef ...

Layer 1-sand bars: a white wriggly line along the coast, where you want the edge of the coral reefs. Paint it with a pretty small soft brush and set the blend mode to screen. Possibly set the opacity to something like 90 or 95%.
Layer 2-lagoons: below the sandbar layer paint the lagoon gradient with a large, very soft, low-opacity, low-flow brush (airbrush mode helps) (probably make a mask to keep it on the right side of the sand bars) - make smooth wide strokes, moving away from the coast towards the bars. Send blend mode to normal or lighten and fiddle with the opacity. You might have to drop a few gaussian blurs on the level, before dropping another ripple effect.
Layer 3-the deep blue: below both other layers draw the narrow, inky gradient heading out. Use a soft, medium sized brush. Set it to multiply and fiddle with the opacity setting. This is a bit like a drop shadow, but make it more of a hint than something very obvious. It's also there to make the reefs pop out a bit more.

I can't try that right now because I don't have my tablet here, but that's how I'd do it .