A free-diver found a military canister bobbing in the waters off Hanauma Bay yesterday morning, forcing hundreds of people to abandon the marine preserve.

An Army explosive ordnance team was unable to detonate the device on the first try and had to be escorted by police back to Schofield Barracks to regroup for another attempt.

The team from the 706th Explosive Ordnance Disposal Battalion transported the phosphorus sea marker to a disposal area at Schofield Barracks around 5 p.m. and blew it up, the Army said in a statement last night.

Kurt Chambers was free-diving with a friend in the area known as Witches’ Brew around 9:30 a.m. when he saw the 3-foot-long, 6-inch-diameter cylinder and swam it into shore.

Chambers, a 30-year-old instructor with Freediving Instructors International, saw a warning label on the canister and worried that a child might play with it.

“It was floating, and with so many kids, anybody could have grabbed it,” Chambers said. “This was a particularly large piece that was encrusted with algae. If it was fresh and clean, I would have thought differently (about bringing it to shore). … It did not appear to be a bomb, and I did not think of it as a bomb.”