The identity of the deceased is not yet available, but Sandusky police said the victim was a city resident.
Sandusky fire Capt. Jim Green said two men in their 20s, including the victim, were on a Zodiac boat driving back to shore at about 6 p.m.
Police said the victim voluntarily left the boat while it was offshore and went into the water but was not able to stay above the surface. The other person on the boat, another man, tried to help the victim but was unable to sufficiently assist, so he went ashore and called 9-1-1.
The call came in around 6:20 p.m.
A Sandusky fire rescue boat and its dive team quickly got to the scene and began to search for the missing man. They found his body at 7 p.m. about 30 yards offshore in 10-foot-deep water, Green said.
They immediately started CPR and the victim was taken to Firelands Regional Medical Center by ambulance.