Picture: https://i.imgur.com/9WFTldz.jpg
You could walk half way down Balboa Pier and still clearly see the bottom of the ocean. I don't think I'd ever seen the bottom of the ocean through waves in any beach in So Cal ever. Never thought of it as dirty so much as just normal for sea water.
Roughly speaking, clear water means less life, less phytoplancton. The algae had gone. Is registered in lakes when water turns more acidic by contamination. They appear pristine, but is an ilusion. The ecosystem is very ill.
At sea this can normally indicate a different water mass by natural processes. Different salinity or temperature, a change on winds, barometric pressure, or sea currents can change this. Is totally normal.
And this could happen also when trowing signifiant amounts of something that kills microscopic life to the water (chlorine to pipelines for example).
Not arguing that they're aren't huge problems with the health of the ocean, but I have seen the water clear up that much a few times a year.
It's really interesting when the visibility is radically different at different depths because of temperature and salinity gradients. Last time I dove it was crystal clear for 15 feet, then a floor of murky green with 2 foot viz, then the clarity opened up about 30 feet down.