The coroner's office was sent to a boat that was docked at Marina Shores in Portage at about 11:16 p.m.
Witnesses on the scene told authorities that a mother had been searching for her son and discovered the bodies of two people in the cabin of the boat.
The Portage Fire Department found toxic levels of carbon monoxide inside the boat.
The coroner identified the victims as 44-year-old Michael McKinney of Wanatah and 38-year-old Jennifer Ellenberger of Crown Point. Both were pronounced dead at the scene.
Authorities said there is no indication of foul play. The coroner's office is performing toxicology tests to determine if both McKinney and Ellenberger died from carbon monoxide poisoning.