If a rental boat, then somebody is in big trouble.
If their own boat, then about four tickets (at least) need to be issued.
The only requirement is for there to be enough floatation devices for all passengers. Now the rental company could make it a condition of rental that they be worn at all times, underage children especially.
And not to nitpick but.. life jackets don’t do much to protect you from going splat on concrete.
Lifejackets wouldn’t have mattered if they tumbled over the edge!