Why yellow sea is called yellow?

The Yellow Sea derives its name from the colour of the silt-laden water discharged from the major Chinese rivers emptying into it. The sea annually receives an immense quantity of sediments, mostly from the Huang He (via the Bo Hai) and the Yangtze River, both of which have formed large deltas.

Sediments are pollutants?

Sediment is a stormwater pollutant that is made up of soil particles that have been detached from the land by erosion

I hope it is still good to swim with those sediments.

Usually it comes from soil erosion. You may swim at other coastlines.