It happens occasionally when there's very little tidal motion (slack tide), not a lot of small particulates from run off or recent rain, and most importantly no

It happens occasionally when there's very little tidal motion (slack tide), not a lot of small particulates from run off or recent rain, and most importantly not a lot of nutrients in the water - most murky water I run into in SoCal is from various species of plankton blooming, and the various zooplankton and copepods that follow in their wake. 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.