According to the U.S. Coast Guard, five people were aboard the tugboat when it flipped over just before 3 p.m. at the 16500 block of Peninsula Boulevard near San Jacinto Boulevard.
Four out of five people were recovered from the water and stabilized.
A Coast Guard helicopter and boats are searching for the missing person with a Harris County dive team.
The USCG credited a good Samaritan with helping recover the four rescued passengers.
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