Underwater robot technologies are crucial for marine resource exploration and autonomous manipulation, and many breakthroughs have been achieved with key indicators (e.g., dive depth and navigation range). However, due to the complicated underwater environment, the state-of-the-art sensing technologies cannot handle all the needs of underwater observations. To improve the autonomous operating capacity of underwater robots, there is an urgent need to develop underwater sensing technology. Therefore, in this paper, we first introduce the develop- ment of underwater robot platforms. We then review some key sensing technologies such as underwater acoustic sensing, underwater optical sensing, underwater magnetic sensing, and underwater bionic sensing. Finally, we point out the challenges of underwater sensing technology and future directions in addressing these challenges, e.g ., underwater bionic sensing, new underwater material development, multisource information fusion, and the construction of general test platforms.