Elaborating on the water edge...

Here is a travel guide picture I borrowed from a quick Google image search to show what I mean:

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I know this is a tropical beach, and the water here is very much more clear than it would be in a stormy temperate zone where there is far more sand being churned up in the water all the time, but it demonstrates reasonably well that the colour of the water tends to be transparent at the edge, allowing the colour of the beach to show through.

On most beaches the hardest line is the strand line, where the high tide pushes all the debris to the top of the beach.

Hope that helps