The 12-foot dinghy flipped in Port Gardner Bay near Everett around 12:45 p.m., tossing two men into the water, Coast Guard officials said.
One of the men was able to tread water for about an hour until a Good Samaritan rescued him and took him back to shore. That man was taken to a local hospital. He was treated and released Monday evening.
Coast Guard helicopters searched from the air for the other victim while police and fire boats looked from the water, but came up empty. Rescue crews suspended the active search just before 5 p.m. and are now focused on finding and recovering his body, officials said.
Later Monday evening, crews said they were going to use side-scan sonar, which can locate small objects under water.
Neither victim has been identified.
Investigators haven't said yet what caused the boat to capsize.