The U.S. Coast Guard says the third person, a fisherman, died after falling from his boat into the water. Blue Island's rescue divers found the man's body in the water a couple of hours later.
Coast Guard Petty Officer Nathaniel Parks says a recreational boat carrying a man and a woman hit a fishing boat after turning to avoid a log. Two men were on board the fishing boat, and one fell into the water.
Parks says the fisherman couldn't swim and wasn't wearing a life jacket.
Authorities say an investigation is under way into what might have caused the boating accident.
(The Associated Press contributed to this report.)