Piracy is an offense to maritime law and an offense to all nations. As such, and one nation or any group consisting of troops from any nation can take back the ship, regardless of it being in Somali waters. Enough time must pass (and it has) to give the hosting nation time to address the piracy. Once it is clear there is no action or intent of action from the hosting nation, it's open season on the pirates, regardless of the waters. That's just basic maritime law, which almost all nations follow.