I've always been interested in one of those European river cruises. It seems like a great way to visit the older parts of a number of cities without constantly packing and unpacking.
I'm less interested in American cities because I've visited more of them. I live in the midwest so many of these ports are just another midwestern city to me.
But, I could see it for one year. It gives my wife a chance to stay in and do her thing while I walk around. (I wonder if there's a place onboard to store a bicycle) If there were a proven resale market, I'd look to see if the numbers work out. As a new venture, way too expensive.