Dive teams searched Friday for a 9-year-old boy and a man missing after a small boat overturned in the swift-flowing Nisqually River, but officials were not optimistic they would find them alive.

"If they went in this water, there's really no hope that they're alive," Dave Pearsall, chief deputy of Thurston County sheriff's office, said at a briefing Friday morning.

The boy's 5-year-old brother died after the boat carrying five people capsized Thursday night just upstream from the Interstate 5 bridge south of Fort Lewis.

Click here for photos.

Divers entered the water at first light, said Jim Chamberlain, Thurston County sheriff's chief deputy of operations.

The boat's operator, Vincent Farler, 42, of Olympia, was arrested for investigation of homicide by watercraft, sheriff's Lt. Chris Mealy said Thursday night.

Farler and the mother of the two boys made it safely to shore after the 16-foot flat-bottomed boat, equipped with an outboard motor, capsized shortly after 6 p.m., Mealy said.

The body of the 5-year-old was recovered about 250 yards downstream.

The woman and her boys were from the Yelm area, Mealy said, adding that neither they nor the missing man were being immediately identified.

Sheriff's officers believe the boat was being operated recklessly and that alcohol was involved.

Apparently no one on the boat was wearing a life jacket. Under state law, Mealy said a boat of that size should have carried a life jacket for each occupant and the children should have been wearing theirs.

"It's one of those things where people don't respect this river enough," Pearsall said. "It may look like a calm river, but it's not. It has currents everywhere, it has undertow. You shouldn't have people out here without life jackets on."

Matt Anderson, who lives nearby and saw the boat flip, said he jumped into the water to try to rescue those aboard but the current was too strong.

Sheriff's officers learned of the capsizing when a citizen reported seeing a woman on an overturned boat, yelling that her sons were in the water, Mealy said.

The river is colder, higher and swifter this time of year because of runoff from the melting snowpack in the mountains. Anderson estimated the water to be about 6 feet deep with an extremely strong current. Lt. Greg Elwin, head of the Thurston County sheriff's dive team, said the water temperature was 50 to 55 degrees, and that people exposed can develop hypothermia within minutes.