Just thinking on how the XMarks Vegas convention fell apart ... Anyhow:
There are Ohio and Mississippi River cruises - I found one site with off-season rates of $600-2000 USD for four-day cruises, with additional discounts for groups over 10 and booking six months in advance.

For a cheaper international option, we could see if there's a river cruise that starts in Canada and ends somewhere in the states. (I would book on that trip just to see my co-workers' expressions when they came to clear the boat: "Sir, there's 18 crew, 120 passengers, and they're all wearing kilts.")

Actually, that reminds me - there are Great Lakes cruises as well. Relatively inexpensive, international, convenient to both Canadian and American XMarkers, the ships aren't bad at all - just a thought.