I got your message and replied. For 20 people, he's going to need an "inspected" vessel. Both Sanctuary, as well as the other boat Pascal runs can only take 12 passenger. "Inspected" vessel status will usually drive the price up since the cost in keeping one at that status can be exponential. Two hours is a short time for a charteryacht. Most day charters are by the day with some offering half days. Whether he wants a half day or full day, he'll likely end up having to pay for the 4 to 8 hours even if he just wants 2. As you know, there is a fixed cost just to leave the slip - captain and crew are hired by the day whether it's for an hour or 10 hours, and if the boat is out for 2 hours in the middle of the day, that kills the whole day for the boat; hence half and full day rates whether you use all the time or not.
The tour boats go out for that kind of short duration, but that will be on a designated route and the boat would be shared with members of the general public - not a true, private charter.
There is a large boat on our commercial dock that is sort of a dinner boat looking thing. She does go out for 2-hour stretches. Her name is Celebration.
http://www.yachtcelebration.com/