Robert Johnston and his crew member Jacqueline Reynolds snagged the explosive two miles off the coast of Kirkcaldy while trawling for prawns.
The 36-year-old believed the net had caught a rock but was unable to pull it up above the waterline on their 29ft boat to check until they arrived at Port Seton harbour.
When they realised the true nature of their catch yesterday morning the coastguard was alerted.
Bomb disposal experts from the Royal Navy’s Explosive Ordinance Disposal Unit were called to make it safe while police evacuated nearby buildings.
Mr Johnston said he believed his boat would have been “blown to smithereens”, killing himself and Ms Reynolds, 31, if the device had been triggered by banging against his boat on the journey home.
Mr Johnston said: “We were out trawling for prawns in the dark when the boat suddenly stopped. We thought it was a boulder, but we couldn’t get the net up above the waterline. We got most of the net up but it was in ribbons really and was at breaking strain.
“We didn’t want to break any more of the net so we tied it up and decided to tow it back.
“It took two-and-a-half hours to reach the harbour then we lowered what we thought was this boulder on to the sea bed. We had to wait until about 10am when the water was low enough but when we went down to take a look we saw all the wiring and realised it was a depth charge.”
Mr Johnston was set to drag the depth charge back out to sea with his trawler yesterday evening after the bomb disposal team had made it safe. The device was due to be blown up afterwards.
Mr Johnston added: “I’m just glad we never realised when we caught it in the net. I would’ve been on tenterhooks the whole time if it banged against the boat or anything. I would’ve been panicky so it was a blessing we didn’t know.
“I’d never see a depth charge before, although I’ve caught a torpedo.”
A police spokesman said: “Lothian and Borders Police responded to Port Seton harbour at around 10.30am yesterday after an item believed to be a World War Two depth charge was recovered from the water.
“A number of nearby buildings were evacuated as a precaution.”