NEWBURYPORT — The family of the man presumed drowned in a tragic boating accident last week is refuting a theory that the boat slammed into the north jetty at the mouth of the Merrimack River, while presenting a very different theory of what may have happened.

Although officials have not yet located the hull of the 36-foot, 1974 Chris Craft or the body of 36-year-old Seth Coellner, Coellner's family suggests the boat came apart after hitting an exposed sandbar in the mouth and not as a result of hitting the jetty, as was initially reported.

Coellner's stepfather, Kenneth Cormier, who is the first from the victim's family to speak publicly, said an eyewitness's account that the boat appeared to be trying to jump the rocks is "ridiculous" and has unnecessarily disparaged the memory of their loved one — a licensed captain and former Coast Guardsman.

"That witness is not credible or doesn't understand boats," Cormier said. "No one is going to make a 36-foot boat jump a jetty."

Cormier said Coellner had logged countless hours in his years on the water, serving four years in the United States Coast Guard and on extended tuna and swordfishing trips on his 31-foot fishing boat in the Atlantic Ocean.

Even with the thick blanket of fog reducing visibility to a quarter-mile, Cormier said the conditions wouldn't have overwhelmed a fisherman with Coellner's experience. In fact, from Cormier's own experience on the water and the stories being told by the three surviving members of the boating party, he's getting a completely different picture about what might have caused the fatal crash.

"I've recounted it," Cormier said. "From what I can gather, I think they lost the bottom of the boat near the jetty."

Cormier said the group overshot the mouth of the river Wednesday night, and after gathering their bearings in poor visibility, they were confident they'd reached the channel and opened up the throttle toward the harbor. From what Russell Hilliard told Cormier, the boat hit something that sent Hilliard over the starboard bow and another passenger through the windshield of the fly bridge and off the port side of the boat.

"They weren't near the jetty when that happened," said Cormier, who said Hilliard would have been thrown onto the jetty if the impact had occurred as officials earlier reported.

"I'm very sure they ran aground. If they had hit the jetty when Russell was thrown from the fly bridge, he would have been on the jetty. The bottom was ripped off the wooden boat by hitting something near the green can at the mouth of the harbor."

Cormier said it's possible the crew hit the sandbar marked with the No. 5 buoy at the mouth. Other officials say it's also possible the boat hit the sandbar farther inside the mouth marked by the No. 7 buoy, and that would explain why divers have so far been unable to locate the ship's hull in the area of the jetty.

"There is a sandbar in there where they might have run aground," said Salisbury Harbormaster Ray Pike, who notes that just before half tide Wednesday night there may have been exposed sand at that site. The accident happened between 9:30 and 10 p.m., about halfway between high and low tide.

"He had plenty of experience, but it is possible to run aground in the fog," Pike said. "They could have been in the Merrimack River while there was still some sand there."

Cormier added that a buoy slightly off station, coupled with the well-known tidal surge at the river mouth could have made it shallow enough for impact even in a higher tide.

"In a surge condition, water breaks when the waves coming in run out of depth," Cormier said.

Pike said this wouldn't be the first time he's seen a boat fall apart after hitting a sand bar at full throttle.

"The bottom could have fallen out on a wooden boat," Pike said. "It's not just theoretically possible. There are boats that fall apart out there based on hitting the bottom."

The theory also made some sense to Tow Boat US owner Michael Goodridge, who is also a salvage diver. He retrieved a large section of the forward deck and pulpit yesterday afternoon and had plans to go out last night to search for the hull using sonar technology.

"That would account for the debris field being further up," he said.

It's expected that many questions will be answered once that section of the boat is located. Coellner was last seen on the main deck of the 1979 Chris Craft, sending out the 911 call that saved the lives of his three passengers: Jacob Clark, 30, of 7 Landing Road, Hampton, N.H.; Russell Hilliard, 50, of 212 Drinkwater Road, Hampton Falls, N.H.; and Mark Baillargeon, 50, of 11 Chapel St., Newmarket, N.H.

Cormier and the rest of Coellner's family are confident that Coellner did everything right and made a "seaman-like" approach into the Merrimack River Wednesday night. They view the incident as an irreversible tragedy that at best might serve as a warning to even the most experienced of boaters who enjoy the sometimes dangerous river.

"They were comfortable with their bearings," he said. "This is the best understanding that I alone have of this."

Responding to conjecture over whether or not drinking might have contributed to the accident, Cormier said that although Hilliard, a passenger, was brought into the station and placed under protective custody, he refutes Coellner was under the influence.

"He wouldn't have left the harbor for the cruise," if that were the case, Cormier said, adding that one member of their party doesn't drink at all and would certainly have protested the decision to go out if those reports were accurate.

"Seth had a lot of miles under him," Cormier said. "He knew what he was doing. It wasn't a ship of fools."