I did five dives (four buff) in two days off Bonaire and Curacao with water temperatures around 80F. On my third dive on day one, I was chilled. On dive two of my second day, I chose to wear my shortie jacket. On both days, I returned to the ship and sat in the sauna for a half hour.

80F is still cold if not actively moving, and after too long in the water your core temperature goes down; you have to build up your body heat again.

Loch Ness would be much colder than our great lakes. It might be ok for a polar bear dip on New Years, but not for any length of time.