Not sure where or how the figures are arrived at, 21ft dayboat on the Broads only allow 10 souls aboard, however that number was decided upon is anybody's guess. A pal of mine owns the Southern Comfort trip boat based at Horning and carry 50 plus, it's twin hulled with a large upper deck, looks like a Mississippi paddle steamer. 60ft long with a beam of about 15/18 feet.
He does a load test by "borrowing" some 1 ton bags of sand from a mate at Jewsons and hoists them to the upper deck with a teleporter or some such, he has to to this for the MCA
Phil