The incident happened at the intersection of Centerline Road and West Foss Avenue in Newaygo County just after 11 p.m.
The vehicle pulling the pontoon boat came into the corner going too fast, and the pontoon fell off the trailer, officials said.
The woman and man were the only two people riding in the pontoon. The woman, who is from White Cloud, was pronounced dead at the scene. The man, who is from Fremont, was transported to Butterworth Hospital on Aero Med.
The man was in critical condition Sunday morning.
The driver of the vehicle pulling the trailer was arrested for driving under the influence.
The accident is still under investigation.