That was the lowest temperature reading in the lake. With milder temperatures, it has settled in at 46F (7.8C). Since the temperature stabilized, I reduced the number of swims to 4 times a week. Surprisingly, it is mentally easier to do the cold swims if you do them every day. This morning I did a mile before work. The "warm up" still takes me about a quarter mile, and after a half mile the cold is only noticeable in the hands. The other peculiarity of cold water swimming is that the most difficult time is just after you get out. Since you aren't exercising, the body heat is no longer being generated and the blood circulation starts getting back into the arms and legs, drawing all the cold into the rest of your body. This is the time when the body really starts feeling cold.
The San Francisco Bay has been running about 51F (10.5C). Thus, I expect the New Year's swim to feel like a warm bath.