[oeuviz]

Due to water being a way better conductor of heat than air you lose heat up to 75% faster than in air. Even 30°C warm water can therefore quickly lead to hypothermia if unprotected. Ever seen scuba instructors in warm countries? While their customers would often submerge in nothing but a swimsuit the instructors would often wear 6mm full body wetsuits due to longer exposure. And yet a lot of them are shivering in the evenings.

[marvin]

I've been shivering in the water at 27 degrees Celcius in Australia (diving, no wetsuit, sitting on the bottom). Admittedly I have a very slender build, but clearly 27 degrees is below my equilibrium temperature when not in activity.

[EGreg]

True, although below the surface the water tends to be colder.

[mixmastamyk]

Yes, it is a lot colder when scuba diving at say 10m, but we’re talking about treading water at the surface.