Having seatbelts actually potentially CAUSED the drowning, since panic can easily set in and cause it all to go pear shaped very quickly... Imagine trying to open a seatbelt after seeing people flung out of your capsized boat!
I always assume that seatbelts come with the layout of the things... Ones with bigger drops in the states have seatbelts, though River Quest and others over here do not... It's probably a mixture of layouts and culture; given that Americans sued McDonalds because the coffee was hot (spilt it over themselves) they need to cover themselves a lot more than other countries do purely over idiocy, having seatbelts means that if anything were to happen to someone like this they can point to having safety standards in place...