Today's (July 12) Groupon.com/chicago deal is a $12/$18 ticket to ride the tall ship "
Windy." Its last cruise of the season is Oct. 15, which is the day before the Do.
That's about a 50% discount.
Caution: It looks as if you have to physically present yourself at Navy Pier to either make an advance reservation, or to get a ticket for the next cruise, so you might have the Groupon coupon but not be able to sail if the ship is full. It's possible that a Chicago resident could take everyone's Groupon coupons down at one time to make reservations, I don't know.