To address the challenges of restricted operating space， variable hull curvature， and increasingly stringent environmental regulations in the underwater maintenance of small special-purpose vessels， this paper systematically reviews and evaluates key technologies applicable to underwater maintenance robots for such vessels. It analyzes the current technical status of underwater maintenance robots both domestically and internationally， with special emphasis on comparing the stability of adhesion mechanisms， namely magnetic adhesion， negative pressure adhesion， and mechanical gripping， on surfaces with variable curvature and non-magnetic substrates. The motion performance of various drive systems under complex operating conditions is assessed， and mainstream cleaning techniques such as brushing and water jetting are summarized. The study reveals the evolutionary trends of existing technologies in adaptive adhesion， high-precision actuation， and environmentally friendly operations. It further explores detection and intervention approaches tailored to non-magnetic composite-material vessels and examines the performance requirements specified in current technical standards for biofouling prevention and pollutant capture. The findings indicate that future underwater maintenance robots for small special-purpose vessels should advance toward AI-based path planning， multi-modal SLAM navigation， and modular multi-functional payloads to achieve efficient， precise， and environmentally sustainable maintenance operations. This work provides a reference for the development of intelligent underwater maintenance equipment for small special-purpose vessels and holds significant importance for enhancing maintenance efficiency of small vessels in China while meeting aquatic ecological protection requirements.