Boston Harbor Cruises, which operates the whale watch, confirmed that the boat strayed into a restricted area, and the company said it was cooperating with a U.S. Coast Guard investigation.
The vessel was freed early Tuesday and docked shortly after 8 a.m. at Boston's Long Wharf. No injuries were reported to any of the 157 passengers or six crew members, though passengers said a number of people suffered seasickness during the long wait.
"Everybody is safe and secure," said Sheila Green, a spokeswoman for the company that operates the vessel. She described passengers smiling and waving from the deck as the boat approached shore.
One of the boat's propellers became entangled Monday about 13 miles (20 kilometers) offshore in a cable in the Northeast Gateway Deepwater Port, the Coast Guard said. The facility handles large liquefied natural gas tankers entering Massachusetts Bay.
The vessel was in a "restricted management area" when it got stuck, Boston Harbor Cruises said. The company said it would defer to the Coast Guard's findings "as to whether or not operator error was a contributing factor to the entanglement."
Divers hired by the vessel's owner succeeded in freeing the boat early Tuesday, Coast Guard Petty Officer MyeongHi Clegg told The Associated Press.
Two Coast Guard cutters remained with the vessel during the night, she said, making sure the passengers were safe and that there were no medical problems.
Ken Maguire, one of the passengers on the boat, said they had expected to be back Monday around 4:30 p.m. but about 10 minutes into the return trip, the boat stopped after apparently hitting something.
Passengers will receive a refund on their $50 ticket, a $100 gift card for a future Boston Harbor Cruise and $500 cash for their troubles, Green said.
The passengers received food and drink and blankets to stay warm, she said.