If you take the L you'll get downtown around 6pm. I'm just max out times for customs, checking in to the Hilton, and the L is a 45 min ride ORD - downtown. Then you'll have to get to wherever you want to take a boat ride. You'll ride during dusk, grab late dinner. Have fun!
Also - as a Chicago native, the blue line to/from ORD is terrible. But enjoy. Haha.