The coldest I have ever been (and ever will be) was when I jumped in the arctic sea. It was 2oC. Which doesn't sound that cold, but it was! They even had a doctor with a defibrillator! We were only in the water for a few seconds, we jumped of a iceberg and then were pulled up onto a zodiac. I even have a certificate titled 'arctic sub-aqua lunatic'! But I don't think I will not do it again! Only one word to describe the experience: cold. Especially when all your wearing is swimming trunks. A bit like this: