Magnetometry plays an important role in exploring the deep sea, which is one of the Earth's final unknown frontiers. However, the complexity of the marine environment and the limitations of conventional magnetometers restrict its in-depth application. The nitrogen-vacancy (NV) center in diamond offers a potential solution to encompass and transcend conventional ocean magnetometers. Its unique advantages, such as precise vector measurement and tolerance to extreme environments, make it well suited for deep-sea applications like navigation. This work introduces the first deep-sea quantum vector magnetometer based on NV centers. The performance of this magnetometer is effectively validated by a series of field tests on the manned submersible Shenhai Yongshi during a cruise in the South China Sea, including an experimental underwater navigation using the diamond quantum sensor as a magnetic compass. This successful deep-sea application marks a milestone for transforming this promising solid-state spin quantum system into a practical sensor for real-world marine applications. The first deep-sea experimental application of diamond quantum vector magnetometer.