How come in Miami Beach (and some others) the water is clear and you can see straight to the bottom,

[1-Old-Man]

5. A whole bunch of reasons, but mostly runoff source

All that South Carolina water you're looking at came across lots and lots of mud (and farm lands) before it got to the beach. Then there is the bottom itself. If you are standing on Miami Beach you are just a hop, skip, and a jump from the Gulf Stream and the clean water it brings up from the south. On a South Carolina beach you will be up to 100 miles away from that same clean water and the bottom between your SC beach and that clean water will be what we fishermen call "mud". The third thing, and much more important in most places than the bottom or the water's passage to the sea, is the amount of living stuff in the water. Put a lot of algae in the water and you won't be able to see through it. Put a lot of nutrients in the water, by making its passage through farmland, and you get lots of food for that algae to thrive on, and cloudy green water to go with it. South Carolina gives you all of the bad things.