Heres some info that may be usefull as spring approaches. It too cold in most places to swim but
San Francisco bay is probably very low 50's and there still..always at it. Lots of swimmers use the thin latex caps over the warmer neoprene caps. That takes a lot of streaching , there are some other options
I visited San Fran for the first time about a year ago....It was cold and I imagine the water was much colder than the wet/windy air. As I ran from fishermans warf to the Goldengate (around the presidio) I was shocked to see the swimmers w/o wetsuits in the bay! I thought I was tuff for getting out and running in that weather. Those people flabbergasted me!
I did my first two swims in the Bay in a neoprene cap (latex overtop) and halfway through the second, ran onto the beach, ditched it, and never looked back. Neoprene cap is just sitting in my gear box getting funky.
It's really not that cold once you get going-- you have earplugs in anyhow (at least I do), so I will take the trade-off for the freedom of movement. Also, usually I'm double-capped anyhow, because I wear a silicone and the groups I sometimes swim with require a specific latex cap on top.