Not certain, but there were a few winters in Inverness where the canal and the river, both of which flow directly from Loch Ness, froze over (the latter only partly), so this suggests some freeze on the loch. However, Loch Ness alone has more fresh water than England and Wales combined, so it gives you an idea of just how cold it would need to be for it to completely, let alone partly, freeze over. Lomond, Morar, Linnhe, Nevis etc are also very big, so I doubt they would freeze over. Small lochan freeze over all the time, especially in the mountains, but it's rare for them to be as frozen solid as they are now with such deep coverings of snow