IMHO it is to cover themselves from drunk passengers that could injure themselves or fall off the boat and drown.
It seems the webite is catering to Europeans and not Americans. I remember a lovely Swiss couple renting the other half of our Club Orient studio and the differences between us were glaring. We were BBQ steaks on the terrace and the smoke apparently was bothering them due to the fact they were vegans and did not drink alcohol. So instead of moving to a different chalet they had to tolerate our drinking and cooking up a storm. They swam to Green Key everyday while we finished a case of heineken. My point is not everyone vacations the same way. That was 1985...