During September, the City of Chicago had extreme rainfall which forced a discharge of wastewater into Lake Michigan. An outbreak of gastroenteritis occurred among persons who celebrated a reunion onboard a dinner boat cruise on September 13. Consuming ice while onboard the cruise was associated with risk of illness. Five patients were identified with the following pathogens: Shigella sonnei (3), Cryptosporidium spp (2), and Giardia spp (3). Shigella sonnei was isolated from an environmental swab obtained from an ice machine. The environmental inspection at the water docking station revealed the potential for contamination of the hose used to load potable water on the boat.