There aren't any charters operating out of Loch Ness, and whilst there are a few shore dives in it - they are dark, can be deep, and not very exiting unless you like peat. It does make a very pleasant few days kayaking from Inverness to Fort William and back however - much better way to spend time. Loch Duich and Loch Carron aren't very far away - both of which have good shore dives - there is a dive charter rib based out of Loch Carron, but never used him.
Water is about 8 deg at the moment - may have sneaked up to 9 deg by the end of the month