In response to Jack C from 2011 (!) the lyrics in the Corries' Songbook do say "Loch Yuma" - which is actualy in Arizona, not Mexico! There are unfortunately other mis-transcriptions in the book, so I wouldn't regard it as definitive.
Loch nan Uamh makes perfect sense however: there was a Jacobite naval skirmish there in 1746 (see Wikipedia) and geographically it fits the story, as Megan and Dr Wimsatt have indicated.