You are quite right that there is no specific point relating to sitting on the tubes.
However in a later more general section there refers to the resposibility to ensure a safe position for all PAX. Now most of us know that in many circumstances the best position is on the tube.
However, and again only our opinion, when stood before His Honour, following a MOB to fatal incident the knowledge of a jurry will lend towards that a safe position is within the boat and not on the tubes. You obviously can counter with statistics etc but would the MOB of occured with inboard seats .. No !
Our policy on our own operation and aour advice to purchasers of our boats is to go for both inboard and tube seating ie. setaing inboard for 12 plus tube handles for 12. In any weather then inboard seating is dictated by the skipper and no one small or young who cannot sit on the tubes but cannot touch the floor should be on the tubes.
In case of accident we would show our transparent policy and above all that we had identified a risk and taken measures accordingly. Then back it up with evidence of hundreds of thousands of people been on our sailings having used the tubes with no incident. ie. we would attempt to demonstrate a genuine accident cum act of God rather than negligence.
If we operated without internal seating for all then I guess we would demonstrate risk control by limiting operating area/speed/weather. Alternatively foot holds etc could be an option.