A 17-year-old youth drowned in the sea off Arnala beach in neighbouring Vasai area Thursday afternoon while four women including his mother and sisters were missing, officials said.
They were bathing in the sea after celebrating Holi.
The body of Prashant Mourya (17) was fished out by rescuers, said chief of the Thane Regional Disaster Management Cell Santosh Kadam.
Prashant's mother Nisha Mourya (40), sister Priya (19) Kanchan Gupta (35) and Shital Gupta (32) were still missing, he said.
Coast Guard had joined in the search operation, another official said.