Hatul
Contributor
Under normal environmental conditions, our body (even at rest) produces excess heat that needs to be dissipated. We typically perspire when the air temperature is well below 35° C / 95° F, attempting to release our extra heat energy. If we’re surrounded by water that hot, we are less able to effectively dissipate energy and could overheat. I think (but cannot find the reference) that thermal equilibrium for a human body at rest immersed in water is right around 34° C, so you could well be in the range of overheating. Unlike in a hot tub when your head is exposed to air, underwater there is nowhere else for the heat to go. So to me it sounds like it could be early signs of hyperthermia. Sensitivity naturally varies with individual.
I like this answer. Normally we sweat to cool ourselves by evaporative cooling, but underwater this obviously cannot take place, so this cooling mechanism does not work. Your body does not know this and continues to sweat, which can result in overheating and dehydration.