Thanks Andy and Alan.
It's interesting to consider exactly what makes something 'an event' rather than an informal gathering - I'm still not sure!
All I do know is that the RIBnet conference Treasure Hunt was 'organised' (to the point of giving GPS waypoints, even) but no-one raised the issue of liability then and I would never have even considered taking legal action against anyone if I had been injured or our boat damaged.
From the experience you recount above, Alan, it seems we cannot even take friends out on the boat without putting ourselves at risk of being sued. Looks like your ribbing days are over then, mate, unless you charter!
Edit: although, I guess, it's our insurance company you would sue rather than Richard or me (as we haven't got any money - house is owned mainly by the mortgage company, two very old cars and that's about it! Oh, I guess you could 'win' Blue Ice!!