Why is the water so cold in California

I'm not from the area, but I visited people a couple years ago near Monterey.

I'm from the East Coast and the Gulf, so when I got there the first thing I said was "LETS GO SWIMMING!"... and was pretty much just laughed at and told that you can't go swimming in the waters there without a wet suit, even though it was the middle of summer.

Is it because of the Trenches? Or are there other reasons why the water is so cold?

In the northern hemisphere, ocean currents go clockwise. This means they head northward along the east coasts of continents, then when they get quite far north, they turn southward and follow along the west coasts of continents. This is why Alaska is warmer than Siberia, and Europe is warmer than Labrador and Quebec, despite the fact that they are at similar latitudes.
So to answer your question, the Humboldt current comes down from Alaska and keeps the west coast of California cool with low humidity.
Perhaps adding a few more billion tons of CO2 into the air will remedy this problem?

I'm just guessing, but I figure the coastline is more open to currents, not enclosed like the Gulf which would allow it to more easily warm up. The Gulf and South Florida is also further south, which would make some difference.

It is really dissapointing that it is so cold. I would like to move their from Sydney but our beaches at 20 degrees celcius. While LA beaches are 14C.