In reply to Paul249:
There's a wild camping spot just off the A86 where the River Pattack flows past and which I'm pretty sure I remember having a swimmable pool under a waterfall.
Aha, it's actually listed on a wild swimming web site:
http://www.wildswimming.co.uk/map/strathmashie/ That web page shows the correct location, as I remember it. A bit more Googling suggests that the place is actually called Pattack Falls, and the OS grid reference is NN 567 903.
My recollection is that if heading towards Dalwhinnie you need to take it slow as you approach the location, and keep a sharp look out for the turning on the right, since it's pretty well hidden coming from the Spean Bridge direction. It's only a dirt track turnoff (unless it's been tarmaced in to a car park since I last laid my head there).
Post edited at 16:14