"In the spirit of public art, Circle Line Downtown and the Public Art Fund are making these tickets available so that everyone has an opportunity to experience the Waterfalls"
Circle Line Downtown is giving away tickets on a first come first serve basis, until the exhibit ends on October 13. The number to call 1.866.925.4631. You can get up to five free tickets. At the time of my call, They had no tickets available prior to Sept 1st. I tried to get some for Friday Sept 5th, in the evening, and they had none at 6.30pm, the last boat out. You can check specific dates and times and see what is available. For instance, they did have 3 tickets available for Friday evening the 16th, at 6.30pm, but only 3 tickets available as opposed to 5. Being a glutton, I opted for a date on Thursday at 6.30pm when five tickets were available.
More info on the 'falls:
(photo of the 'falls from flickr user tais)