Seven fishermen were "fortunate not to be killed" after their boat was hit by an underwater German wartime bomb.
Five crew members on board the Galwad-Y-Mor suffered significant injuries - some life-changing - after the explosion on 15 December off the Norfolk coast.
A Marine Accident Investigation Branch (MAIB) report into the incident said: "Although the physical injuries were significant to five of the seven crew, they were fortunate not to be killed."
The fishermen had been hauling a string of 100 crab pots from the seabed when they noticed "a lot of tension" on the main line, the MAIB report said.
At around 11.22am the skipper revved the engine to break the line free, when three loud bangs resounded on the main deck.
The movement had disturbed an unexploded 250kg bomb dropped in the sea during World War Two, with the ensuing shockwaves sending the boat rocking. The skipper hit his head and was dazed while four of the crew were severely injured, but all remained conscious.
The MAIB report said it was a German-made high-explosive - probably an SC250, a bomb widely used and feared during the Blitz.
Power was immediately cut out and water began flooding the deck. The skipper roused the night watchman and sent a distress call to the coastguard, while the crew were ordered to prepare to abandon ship.
The captain of another nearby vessel, the Esvagt Njord, heard the distress call from around four miles away and sent a rescue boat to help save the crew.
The fishermen were hoisted on board the Esvagt Njord and given first aid, including from a paramedic.
Two were airlifted and taken to hospital at around 1.50pm, while the others were taken to hospital later in the afternoon.
A shard of metal later found in a crab pot was sent to specialists for forensic examination.
Analysis suggested to "a high degree of certainty" that it came from an SC250, the investigation found. The unexploded bombs remain highly volatile, even after many years underwater.
The report concluded that the skipper and crew "could not have foreseen the explosion and their level of preparedness to deal with such an emergency saved lives".