Trump vs. China: US Underwater Drones Used for Oceanographic Explorations Illegally in South China Sea?

According to reports, US President-elect Donald Trump slammed China for allegedly "stealing" an oceanographic device owned by the US Navy straight off the water. Many people were appalled by how Trump impulsively reacted to the incident and accused China of stealing, creating even more tension between the two countries. But were these U.S. underwater drones legally placed in the South China Sea?