I just got back on Sunday after spending a week there with record setting lows. I dived Wednesday and Friday. At depth the temps on the reefs were 72F and on the Vandenberg it was about 74F because of the warmer Gulf stream. Wednesday I had on an isotherm layer, a full 3ml, a shorty over that, gloves, booties, and a hood. On Friday I used the isotherm layer and a 5ml with hood, gloves, and booties. It's not the water temp that will get to you, it's the wind chill once you're out. I was fine during the first dives of the day but couldn't warm up on the surface interval (it was about 54F with wind chill in the lower 40s) on Wednesday. It was incredibly uncomfortable and I was probably close to hypothermia (teeth chattering uncontrollable, lips were blue).
Last Friday Key West finally had a warm-ish day and I was comfortable on both dives. Bring at least 2 dry towels: 1 for after first dive, 1 for after 2nd, and bring dry clothes and a ski cap to change into on the boat. That helps immensely. If you have a thermos you can fill it with a warm drink to thaw out your core. A word of advice: if you want to have warm clothes like a ski cap and scarves, bring them with you. It was almost impossible to find anything like that on the island.