I finally (after 2 hours of sleuthing) tracked down an itty bitty description that said the crew swam to Fitzwilliam Is. which I eventually found on a map somewhere else. And in a third place I found a description of the relative positions of Fitwilliam I. to Perseverance I., so I have a high confidence level (but not 100%) that I actually located the island and the wreck.
Phew.
If you're interested, here's the
web page with my map and my sketches (yes, sketches) of my dive weekend: