At one of my previous off sites someone had a 4x4 atv roll on them. Spent a month+ at a hospital before he could travel home and then had months more of recovery.
edit: Remember companies, always make sure there's insurance covering the event and activities.
On an offsite for a startup, we went canoeing on a river. That can be really dangerous. My canoe flipped and sank. So I stupidly dived down to retrieve it and succeeded in putting my life at risk. You can get trapped under water in a river quite easily. I did but I was strong enough to get myself untrapped. I'd rather just program, thank you.
Less 200 people die canoeing in the US every year [0], so I don't think it is particularly dangerous. Surprised your canoe sank though, usually they are supposed to have air pockets so they float even if full of water.
Canoes don't typically have air bags unless they're outfitted for serious whitewater but they certainly shouldn't literally sink even if they fill up with water.
I have trouble imagining the sort of canoeing I'd expect to see as part of a company outing being some life-threatening activity.
(That said, I'd probably avoid this sort of thing if I were organizing a company event. People have different ranges/types of activities they like and feel comfortable with and I wouldn't want to put pressure on people for what's ultimately supposed to be a fun event. I know whenever we've discussed doing anything along these lines as part of an offsite whatever ideas we come up with always get some percentive of strong "hell nos." Doesn't see worth the trouble.)
I actually got thinking about Grumman aluminum canoes which are obviously denser than the equivalent volume of water. Apparently they do have some flotation built in.
edit: Remember companies, always make sure there's insurance covering the event and activities.