There are some days in the winter it gets chilly but that didn't stop the surfers, they were out there in wet suits. It is almost always sunny (hence "Sunny San Diego"). The only thing I would say if you have never been there is to watch out for the kelp. It can wrap around you and grab you and cause an unfun experience. The water out there isn't very clear around the shore so snorkeling wasn't as good as Hawaii.

I would recommend the southern beaches by Mexico, I think the one I used to go to was Imperial Beach.