A superyacht carrying about 8,000 litres of fuel sank after catching fire on Saturday off the south-west English resort town of Torquay, raising pollution fears.
Firefighters battled to extinguish the blaze on the 26-metre vessel moored in a marina.
Emergency services declared a major incident and officers evacuated a nearby beach and roads.
One witness describing the blaze as “like a fireball”.
The Devon and Somerset Fire and Rescue Service said the fire was put out by around 6pm and the matter had been handed over to the Environment Agency and the harbourmaster.
Police, who were alerted about the fire just after noon, said the cause of the blaze was being treated as unexplained and inquiries were continuing.
Video on social media showed black smoke and flames enveloping the anchored white yacht.
No injuries were reported, but the harbour was temporarily brought to a standstill, with fears over a large amount of diesel fuel on the boat.
A fire services representative said the yacht contained “approximately 8,000 litres of diesel fuel”.
Local media put the value of the vessel at about £6 million ($7.6m), but it was not clear where this figure came from.
The Environment Agency South West said the focus had shifted to “pollution response” after the boat went down.
“Our officers have been checking air quality around Torquay harbour this afternoon following the yacht fire, which was declared a major incident,” it said on Twitter.
“Thought to be no real issues with air quality, but we're keeping an eye on the situation.
“With yacht now sunk, and with tonnes of diesel on board, focus now on pollution. Pollution response led by harbour master. Our officers are helping with advice and guidance on bathing water quality et cetera.”
Grace Kedzior-Macdonough, 20, was visiting the seaside town with her partner when she heard “bangs or explosions” coming from the harbour.
She told the PA news agency she saw crew in neighbouring vessels spraying hoses to stop them catching light as the fire was “spitting” flames out on to the water.
The ropes mooring the superyacht then burned off and it detached from the harbour and drifted towards a bridge which acted as a barricade, she said.
“We were just confused at first because we were just sat round the harbour and there was just this black smoke coming from nowhere.”
Musician David Balmer, 58, said the fire drew a crowd of onlookers.
“There were masses of people around. It was engulfed in flames and there was this huge cloud of black smoke,” he told PA.
“It was quite a spectacle, it was just an unbelievable sight.”
He said people were told to leave the area because “there might be an explosion” and the crowds were moved to a safe distance.