Marketing photos and videos, posted for years by the company advertising the rides, show people not wearing life jackets. Pace of the Highway Patrol said state law requires that the vessels have one jacket for everyone on board and that the jackets be readily "accessible," not stowed under a seat.
This video from Ride the Ducks social media page about two years ago, for instance, shows a packed boat of tourists on a leisurely ride on calm water. Not a single person featured in the video is wearing a life jacket.
I don't think it would have helped much. The vessel was an amphibious truck with a roof and windows. Wearing a life jacket in an enclosed vessel that has capsized makes escaping that vessel nearly impossible, as the swimmer cannot submerge in order to escape.
The real culprit here, in my opinion, is the Captain of the vessel who did not bother to check weather conditions before taking passengers out on the lake. The storm that caused the sinking packed winds at the upper end of Tropical Storm force, and warnings had been issued for the weather before the boat sailed.
When you are in an area where storms are frequent, like the Midwest, it is easy to become complacent and ignore storm warnings. In this case, the Captain's complacency cost 17 people their lives.