From what I can tell most or all other significant mishaps with these vehicles have involved them sinking: In 1999 in Lake Hamilton in Arkansas where 13 died; Off Milwaukee in Lake Michigan in 2000 where all crew and passengers where rescued; and In the Delaware River in Philly in 2010 where two passengers died. Surprisingly to me, I couldn't find anything about where they've been involved in any other fatalities on land as, from what I've witnessed here in the other Washington, they seem to have a tendency to bounce around a lot when going down the road.
I saw one over the summer in San Francisco. However, they have discontinued the tours since they would have been subject to a new city ordinance that would prohibit the driver from also serving as the guide. They closed shop since they claimed they couldn't justify paying for a separate narrator, who would also take up a seat.
The law in San Francisco is for all tour operators, after a cable car styled tour bus (run by the same company that ran the SF Ride the Ducks business) hit a pedestrian (who died from it).