In 2008 I did the first* Blackrock to Cobh 8 mile (tide-assisted) in October without a wetsuit that took me 3 hours in 12 to 12.5 Deg Celsius. I had already done a couple of similar or longer distance swims but not at this temperature continuously. (I had done Clew Bay at 12 miles and at a similar temperature for the first hour and a half, but swimming into a river estuary which gradually raised the temperature.)

I was expecting about a degree warmer. There were 14 swimmers some with wetsuits, some without. I was the thinnest without a wetsuit and the last exiting the water.
It was quite cold at start, for maybe 1 minute, I felt OK after a few minutes. I swam fine for the first 50 minutes, when I had my first food break (a warm drink). My hands never regained full flexibility after that and they gradually lost efficiency.

My fingers were spreading at 1 hour. I had warm drinks about every 45 minutes after first break. After coming out of the Passage Channel and around Haulbowline, the last mile was horrible with wind against tide, lots of chop, very shallow in places, and I was really struggling. Support kayaks were checking me for the last hour, to see if I could the remember day of week etc, simple cognition and speaking tests. My hands were completely frozen and clawed, and my arms numb to my elbows.
I was “Mildly Hypothermic” for 15 minutes after the finish. With my fiancée’s assistance, I was able to get dressed but I don’t remember anything for those 15 minutes though I <was> functional. My girlfriend says I was coherent but speaking extremely slowly, taking seconds per word. Lots of layers and warm drink to warm up. I don’t however recall any serious shivering but I’m not saying there wasn’t, only my memory isn’t reliable so I think there must have been. I guess it took me a hour to get comfortable, and maybe another hour to feel ok. I’m sure you noticed the word Mild there. There are various states of hypothermia, mild to severe. Mild is body temperature of 32 to 35 C. (36.5 to 37.5 C is normal). At Moderate you are turning blue etc. Mild Hypothermia is a lot more than just being bloody cold!

That was a very valuable experience and useful information about my own limits. Had I done the same swim last year, I think I’d have checked the water temperature first. If it was the same….I’d have worn a suit, I’d learned what I needed.

*(Renewed) As this was originally first swam by Coach Eilish when she was 14 years old!