The fishermen carried the explosive device from the sea to the shore near the historic Fort Jesus, in the hope that they had found valuable scrap metal.
It was only when they reached ashore that they realised they had a lethal device on their hands and decided to contact the police.
The police, led by Mombasa urban police chief Kipkemoi Rop, said they suspected the explosive dates back to the 1940s.
“It’s not a new bomb. I suspect it was of 1948 during the (World War II). The bomb is not of the new model of bombs,” Rop told journalists.
“Fishermen removed it from the sea and thought it was their time to make money from scrap metal dealers but realised it was a bomb and decided to call the police,” the police chief added.
Bomb disposal experts and Kenya Navy divers were called to the scene and opted to detonate it at sea.
Kenya Navy Senior Sergeant Malonye said they decided to detonate it at sea due to public safety.
He said the explosive device was capable of affecting a radius of about 3km if detonated on dry land.