We had an Illuminations cruise in 2002. We paid for the boat the night of the event. Things could have changed since then, but we tipped about $25, which at the time was about 20% of the trip.
Nice relaxing trip. MY DS at the time was 4 yo. It was a full moon. He stood in the front of the boat and "directed" the music. At the end he turned to me and asked "Mom - Did you like what the Moon and I did for you???" Talk about priceless. What wasn't priceless were the guys peeing off the boat next to us. Have fun!!!