Vu Thanh Ngu rented a pontoon boat Sunday intending to spend a leisurely day with his family and friends searching for scallops in the shallow waters off Citrus County.
By the end of the day, rescuers from three agencies were searching for him instead.
On Monday, rescuers continued to search a 5-mile area around channel marker 10 near the mouth of the Homosassa River, the last place the 37-year-old Riverview man was seen snorkeling.
Rescuers suspended the search after about 10 hours Sunday because of darkness, but resumed their efforts about 7 a.m. Monday. Citrus County Sheriff's Office spokeswoman Heather Yates said her agency is now investigating the incident as a drowning.
Ngu had rented the 20-foot pontoon boat from River Safari in Homosassa. At Ngu's Riverview address on Monday, a man opened the door and said, "We are very sad right now. We don't want to talk."
Witnesses said Ngu was in the water with a safety line but he lost hold of it and started having trouble swimming, according to sheriff's officials. Others on the boat got in the water and tried to help him, but Ngu did not resurface.
On Monday, a sheriff's dive team along with the Coast Guard and Florida Fish and Wildlife Conservation Commission officers searched from the water and from the air.
Although winds and rough waters that hindered rescue efforts on Sunday had dissipated Monday, Yates said rescuers were still dealing with strong gulf currents.
"That's making it more of a challenge," she said.
Times staff writers Danny Valentine and Sarah Hutchins contributed to this report.