Wow, We were just here and just on this very same boat ride. I wrote about it here: http://blog.bonelakesoftware.com/2011/02/hiking-in-iguazu-national-park.html. Now today I find out that two American tourists have died doing that same excursion.
Apparently the boat hit some rocks and flipped over. With the power of the falls and that river its amazing that only two people died. http://www.smh.com.au/travel/tourists-killed-as-raft-flips-under-iguazu-falls-20110322-1c4f9.html?from=smh_sb Its worth following the link to the article as it has an awesome picture of a rescue working being lowered from a helicopter that gives you a sense of the scale of these falls.