Huge draw? Your definition and mine must be different. I'm not saying it wouldn't be unique and interesting, but I hardly think it would draw a ton of people. I can think of maybe 3 people in my life that would ever even consider going on something like this and I know a lot of people that are outdoor enthusiasts.
I put this in the same category as running commuter rail to Hastings. Just because 3 people might use it does not make it a good idea. I just don't see where this would be successful without being 100% subsidized, then at that point do I want to see my tax dollars going to a guy taking the day off to go on a raft ride?
When it comes to rail you have to start somewhere. It is almost guaranteed that the first couple of train lines will not have enough ridership. That's why you create a network of train lines. Lets take the Northstar line for example. Right now they can go downtown. Expect ridership on the Northstar to increase when they can switch trains and get to the UofM.
I for one believe that increasing the size of freeways to handle traffic for suburbia is a waste of money. I also believe that wars in the middle east to keep fuel affordable to suburbanites is a waste of money as well.
Continuous wars in the middle east for oil and increased prices. Or building a cheaper alternative. Which is trains.
When it comes to this whitewater park, however. You need to have many things to do downtown for people to go there. This will be one of the many choices. How many people would eat at a restaurant in Minneapolis after whitewater rafting. I would gather that people get hungry after doing physical activity.
You have to see the the economy is interconnected in many ways. While this in itself "might" lose money it may bring enough people downtown spending money at other places to be worth it.