It's a process known as upwelling, which plays out along the entire U.S. Pacific coast. Upwelling explains why water temperatures along California, Oregon and Washington beaches are much cooler than similar-latitude beaches in the East.
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What does Earth's rotation have to do with ocean temperatures off San Diego? The average water temperature off Scripps Pier in September is 66 F. Follow San Diego's line of latitude (32.7 N) over to Charleston, SC, and you will experience an average water temperature of 82 F.
Water temps are marginally higher on the east when compared with western beaches of the same latitude. Climate boffins reckon this is because ocean currents swirl in a clockwise direction in the northern hemisphere, pulling warm water in from the Equator.
The upwelling further cools the already cool California Current. This is the mechanism that produces California's characteristic coastal fog and cool ocean waters. As a result, ocean surf temperatures are much colder along the Pacific coast than the Atlantic coast.
That is true because ocean waters rotate in clockwise fashion. This means Atlantic waters travel to the U.S. East Coast from the south and are relatively warm compared with West Coast Pacific waters that arrive from the north out of the chilly Gulf of Alaska.
The current runs through northern parts of the Pacific Ocean and cools off. It then runs down the west coast of North America making the beaches cooler. On the east coast of North America, the Atlantic coastline current is basically warm water on it way north.
Along the east coast of the U.S., the Gulf Stream carries warm water from the equatorial region to the North Atlantic Ocean, keeping the southeast coast relatively warm.