Here's what I did... Prior to swimming I tried to keep my body warm. This started when I got out of bed, immediately putting on sweatpants and a sweatshirt, even though the house was comfortable. Then I kept the heat up in the car on the way to the swim, and wore a puffy jacket and hat until the moment I put on my swim gear.
For the swim I wore two caps (one silicone, one latex, that's just what I had on hand) and my booties and gloves. I swam for 25 min or so, to the second stair case. And I felt comfortable the whole time (OK, the cold side of comfortable, but still ok).
My one mistake came near the end of the swim. I started chatting with another swimmer and just gently paddling. This got me COLD. My fingers started chilling first. I tried to make fists inside my gloves and ended up removing the gloves, which just made my fingers colder.
For the first time, I did have a small amount of the teeth chattering after the swim. This only lasted about 30 seconds and I was able to control it. But that is the threshold I don't want to cross. I think I would have been ok, and not had the chatters, if I'd not done the slow paddle chat and instead finished swimming. I'll know for next time!
Air Temp- 40. Water Temp- 48.