Someone stepping in it will start to sink. To move within the quicksand, a person or object must apply sufficient pressure on the compacted sand to re-introduce enough water to liquefy it.
In actuality the quicksand itself is harmless on its own, but because it greatly inhibits human locomotion, the quicksand would allow harsher elements like solar radiation, the sun, or tides to harm a trapped person. Discovery Channel.
As the bay is very broad and shallow, a person trapped by the quicksand would be exposed to the danger of the returning tide, which can come in rapidly.
Source: Wikipedia > Quicksand
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