1 Cameraman: {shivering with a blanket wrapped around him} G-g-g-g-gg-g-g-...
2 Jamie: This website says it’s inadvisable to go swimming in Loch Ness.
3 Jamie: Not just because of the monster. Mostly because the water’s so cold that you’ll get hypothermia very quickly.
4 Adam: Myth confirmed!
4 Cameraman: B-b-b-br-b-bbr-b-...

The helpful website Visit Inverness/Loch Ness has a very useful page on wild swimming in the Scottish Highlands.^[1]

It says, and I quote:

First things first, we do not recommend you go wild swimming in Loch Ness. Apart from the small matter of Nessie lurking deep beneath the surface, the water is bitterly cold all year round – only around 5°C. In these low temperatures, you will quickly get hypothermia. So, in other words, wild swimming in Loch Ness is very dangerous! However, during the summer months, some of our much shallower lochs and lochans in the area warm up considerably, as high as 15°C. This can make for some very pleasant wild swimming, wearing a wetsuit.

There are several take-aways from this. Firstly, an official tourism website for the Scottish Highlands admits that Nessie, a.k.a. the Loch Ness Monster, is a real danger to swimmers in Loch Ness. I guess that settles that question.

Secondly, as someone who lives in a city where it's reasonable to go swimming in the ocean at any time of year, the idea that some of the much shallower lochs warm up to a staggering 15°C in summer, making them "very pleasant" to swim in while wearing a wetsuit is enough to make me never want to touch a drop of water in Scotland, ever.

[1] I'd never heard of "wild swimming" before, but the context seems to make it clear that it refers to swimming in natural bodies of water, as opposed to artificial swimming pools. I get that it's useful to have a nice pithy term for "swimming in natural bodies of water", but I never would have picked "wild swimming", which sounds like the sort of thing that involves crocodiles and that only Tarzan would really do.