Not boating related, but still a towing incident.
Had just purchased a GSXR-1100 years ago, and was towing it back from the dealership using my friend's Bronco. Driving down Rt. 21 in beautiful Newark, NJ, I'm going around a bend in the road and glance over to see my own bike passing me, trailer and all.
Turned out my buddy had lost the pin to his receiver, so he just used a large nut and bolt in it's place...which wasn't as strong as it needed to be.
All turned out ok, since the bike and trailer slid along and slowly coasted over to the curb. Nobody hit it, the bike stayed on and upright, and the only minor damage was to the trailer.