Fishermen hauling in their equipment set off a Second World War bomb on the seabed, causing an explosion that almost sank the boat and injured five crew, a report found.
The 15-metre long Galwad-Y-Mor was hauling in crab pots off the Norfolk coast on 15 December 2020 when it disturbed the 250kg bomb, triggering shock waves that threw the vessel about.
Five of the seven crew sustained significant head, back and knee injuries, and the boat's hull and machinery were badly damaged, according to a Marine Accident Investigation Branch (MAIB) report into the incident.
Despite that, they were able to set out a distress message and launch the liferaft, eventually being picked up by rescue boats sent from a nearby support vessel.
The injured crew were taken to hospital by helicopter and RNLI lifeboat. The boat was successfully salvaged, towed to Grimsby and has since been rebuilt.
Analysis of the boat's hull showed that the Galwad-Y-Mor had not been directly hit by fragments of the bomb, said the report.
"It is likely that direct exposure to the full detonation of ordnance, containing 123kg explosive charge, would have blown the vessel apart," it said.
"Although the physical injuries were significant to five of the seven crew, they werefortunate not to be killed."
The report also identified two key safety issues for others to learn from.
The first was that unexploded ordnance remain highly volatile even after many years underwater, while the second was that the boat's crew could not have anticipated disturbing the bomb with the strings connected to the crab pots.
Their training, experience and emergency preparedness improved their chances of survival, it concluded.
"The aim of this report is to highlight the dangers that still exist with unexploded ordnance in the seas around the UK, and the actions to take should fisherman encounter any.
"In this case, the skipper and crew could not have foreseen the explosion and their level of preparedness to deal with such an emergency saved lives," it added.