A sword believed to have belonged to a crusader who sailed to the Holy Land almost a millennium ago has been recovered from the Mediterranean seabed thanks to a sharp-eyed amateur diver, the Israel Antiquities Authority has said.

Though encrusted with marine organisms, the metre-long blade, hilt and handle were distinctive enough to notice after undercurrents shifted sands that had concealed it.

The location, a natural cove near the port city of Haifa, suggested it had served as a shelter for seafarers, said Yaakov Sharvit, director of the authority's marine archaeology unit.

"Actually it's heavy", Sharvit said while holding the sword and talking about the Crusader knight who had probably fought with it.

"I'm trying to imagine him on the field with all the armour on him and the sword and fighting with it ... maybe they were bigger than us today but definitely stronger. And it's amazing," he said.

The sword, believed to be 900 years old, will be put on display after it is cleaned and restored, the authority said.