A financial stake in the ocean, or "financial waters", relates to a country's rights to whatever natural underwater resources are found in its oceans.
The government has been apprehensive about such a move in the past, fearing it may cause diplomatic incidents with its neighbors.
The decision to go ahead and redefine "financial waters" was prompted by Egypt and Cyprus claiming stakes of their own in the Mediterranean.
The Justice Ministry's memo does not include the waters off the coast of Gaza in the future financial waters, but sources in the ministry estimated that when such a stake is claimed, it may include some of those waters as well.
Regardless of the final claim Israel may lay to any underwater resources, sources in the Justice Ministry predicted that any such law would attract harsh international criticism.
Tova Tzimuki contributed to this report