I recorded the sound using my mobile, which is how I came to drop it in the river. It is not waterproof but I managed to grab it after only a second or two in the water. Switched it off and after two days drying it worked fine.
I made light of the problems. The launch site was a 1 metre diameter 'hole' in extensive tree cover. Being at the bottom of a ravine, I got zero satellites and launched in ATTI mode. At about 5 metres I hovered and got enough satellites to switch to GPS. The overhanging trees were a concern but the biggest worry was the spray which I could only guess at 'safe' zones close to the falls.
We live and learn. I have a happy ending and I now know what adrenaline smells like!