A boat that hit something under water and dropped a group of fourth graders into Chesapeake Bay had ventured into a prohibited area, a U.S. Coast Guard official said Thursday. The U.S. Navy warns of unexploded bombs in the vicinity.

Everyone on board was rescued after the 40-foot Chesapeake Bay Foundation vessel sank near the mouth of the bay Wednesday evening, in water about 10 feet deep, just west of Bloodworth Island.

Foundation President William Baker said the Coast Guard informed him Thursday that the experienced captain won't lose his license to operate under Coast Guard authority. The Coast Guard declined to comment on the specifics of its investigation.

The area is marked marked off-limits on nautical maps because of underwater obstructions, Coast Guard Petty Officer 2nd Class David Marin said.

Hitting those obstructions could have explosive consequences.

The Navy used the Bloodsworth Island Range for live-ordinance training from 1942 to 1995, according to the Naval Air Station Patuxent River, which warns against trespassing too close to shore.

A Coast Guard investigation may determine just what the vessel hit.

Baker said the foundation has led trips into the general area for years because it provides a sense of what the bay was like before the nation's largest estuary became seriously polluted.

"It's one of the most beautiful parts of the Chesapeake Bay," Baker said.

The foundation said 23 people including 14 students were picked up by local boatsmen. All wore life jackets, and sat on top of the vessel's submerged canopy while they waited for help.

Five people suffered bumps and bruises, and all were taken to hospitals as a precaution before being discharged, the foundation said.

Baker said students he spoke to at the hospital told him "things were fine, and all of a sudden the boat stopped."

The Coast Guard received an alert with a GPS locator and a distress call: "Mayday, mayday, mayday. This is the motor vessel Karen N. We have sunk off of Bloodsworth Island." Then communications were lost.

"People responded from everywhere, but it's hard to get to Bloodsworth Island," said Maryland Natural Resources Police spokeswoman Candy Thomson said.

A nearby commercial fisherman, Jeremy Shockley, arrived at the scene before the Coast Guard boat and helicopter crews. He took them all aboard the Lady Ka Kee and brought them to shore in Wingate, on Maryland's Eastern Shore.

Shockley told WBAL-AM that it looked like they were "sitting on top of the water." He marveled at how calm the children were as his crew helped them off the canopy.

"It was unreal how the kids acted," Shockley said. "They all had life jackets on."

Shockley believes the boat hit something the military sunk in the area, and said he was worried his boat might strike something as well.

The students from Kent School in Chestertown, Maryland, were on an annual science trip, said the school's admissions director, Tricia Cammerzell.

Baker told The Associated Press at the foundation's Annapolis headquarters that the boat's captain, Shawn Ridgely, is Coast Guard certified and has a "stellar reputation," with an unblemished record during 10 years of foundation trips.

The boat was well below its passenger limit of 32 people, he added.

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Associated Press writer Sarah Brumfield contributed to this report.