The 47-year-old man was reported missing shortly after 4 p.m.
The Coast Guard from Station Grand Haven and marine patrol from the Ottawa County Sheriff's Department, already on patrol, were on the scene within minutes after the call.
A Coast Guard official said a search for the missing man is being conducted about three miles north of the Grand Haven pier heads and one mile out in the lake.
An Allendale man who was on the boat, and who declined to be identified, said they stopped the 37-foot Sea Ray at the man's request so he could go for a swim, despite their warnings about how cold the water was.
There were 11 adults on the boat, the Allendale man said.
"We just went for an afternoon with friends," he said.
The people on the boat threw life rings and ropes to the man when he started struggling, the Allendale man said. They saw him go under.
The Allendale man declined to say anything further, and police were not available for comment, so it is unknown if anyone from the boat went in the water to try to help.
The missing man worked for a Grand Rapids ambulance service and taught CPR in high schools, the Allendale man said.
"He was the nicest guy in the world you would want to meet," he said.
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