We summarize key technologies of underwater target exploration in more details, including underwater image preprocessing, target detection and recognition, and target tracking algorithms. First, underwater image preprocessing algorithms are divided into image enhancement and restoration by judging whether they need to build a model, and then we discuss the basic ideas and characteristics of enhancement and restoration. Besides, the principles and methods of target detection and recognition, and target tracking for underwater optical images are reviewed across the board. After summarizing and analyzing the research results of the above processes, we untangle the core issues to be addressed and related difficulties existing in key technologies of underwater target exploration, and discuss the solutions and further development directions as well.