Cold water is only a problem the first few minutes. After that, it is refreshing. You find that out every single time you gave it a go and stayed. But when you don't, or the one time you went in and out immediately, are the times you remember when you don't have enough data points where you stayed.

That cold water is partly responsible for California’s perfect weather. A warmer sea would produce some undesirable weather phenomena like hurricanes.

The water is indeed cold most places here. But once you get down to Santa Barbara or better yet Long Beach you can find the occasional beach with warm water.

It's some combination of the very deep water being rather near shore and beaches that get directly blasted with the cold water streaming down the coast from Alaska. Remove/minimize those 2 factors and you can start finding warm water.

In San Diego around late August / early Sept you can find some really warm water relatively speaking. Particularly I used to enjoy going down to La Jolla cove and snorkeling around that time, with temps reaching up to 75f, which feels like bath water for the Pacific Ocean. Fairly certain I've experienced warmer than that a few times growing up.