The boy was swimming in the lake on Sunday afternoon as part of an Everett church outing, police said. Witnesses called 911 when the boy went missing in the water.
Monroe firefighters found him, performed CPR and rushed him to a local hospital.
He later was taken to Seattle Children's Hospital, where he died just after midnight Tuesday.
The boy's name has not been released.
It was at least the second known drowning death of a child in Snohomish County this summer.
Jeremiah Monroe Jr., 12, of Everett, died Aug. 8 after being pulled from Martha Lake near Mill Creek several days before.
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