Authorities have identified the man who drowned as 31-year-old Jose Rubio.
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Two Chicago Fire Department divers pulled the two men out of the water near Diversey Harbor around 3:30 p.m. Saturday.
At a press conference Sunday, Chicago Fire Department Marine & Dive Operations Deputy District Chief Jason Lach said the men jumped in to swim, while a third person stayed on the boat. The water and wind conditions suddenly changed, separating the men from the boat, which had not been properly anchored and subsequently began to drift. There was approximately a quarter mile between the boat and the swimmers.
"Waves picked up considerably. Winds picked up, pushing them around," Lach said.
WATCH: Chicago Fire Department officials give update on Saturday's drowning and water safety on Lake Michigan
CFD press conference on Lake Michigan drowning
A third person on board the boat was inexperienced at navigating the boat and was unable to drive it.
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The two men in the water tried to swim to shore but were unable to and became unresponsive.
Lach said the boat was about a half mile from shore. Authorities later helped tow the boat to Belmont Harbor.
Rubio, a husband and father of three, was taken to a hospital and pronounced dead. The 28-year-old swimmer, who authorities and family identified as Rubio's brother, was transported to a hospital in stable condition.
Lach explained that the weather on Saturday started out with very calm water and hot temperatures. Then the weather and storms came in from the north, which made the lake very choppy and difficult to navigate.
At least 20 people have drowned so far this year in Lake Michigan. Chicago Fire Department officials emphasized Sunday how important it is to take the lake seriously, particularly this summer as higher-than-normal water levels elevate its dangers.
"Boaters out here need to know how to operate their vessels, they need to have safety equipment on at all times and know to pay attention to weather conditions; they can change rather quickly," Lach said. "Lake Michigan is just a small ocean."
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Just as crews were wrapping up, divers had to get back in the water for another rescue.
"The water conditions changed. The lake rose coming into Diversey Harbor which can be dangerous. Boat lost control, hit the side wall," Lach said.
They jumped in and pulled the man out of the water quickly.
"We are always ready, anytime, any place," said Lach.