If it's international waters, the Chinese don't need anyone's permission to pick up an unmanned drone either. 

To be lawfully immune from interference, a sea-going vessel must be manned. Otherwise it can be considered abandoned, or flotsam or jetsam, and retrieved by another ship. Ordinarily no one interferes with drones in  international waters when they're used for scientific or commercial purposes. But the Americans were obviously spying on the Chinese, so the Chinese picked it up. For all the sputtering outrage from Trump and others here, if the roles were reversed the Americans would've done the same thing