This paper investigates the motivations for developing underwater gait assistance robots and explores methods that leverage the benefits of partial submersion for rehabilitation. An overview of the current state of underwater gait rehabilitation robots is provided, highlighting the successes and limitations of existing designs. The study surveys key approaches in gait characterization, actuation, sensing, system modeling, and control strategies to identify those best suited for underwater environments. Emphasis is placed on how buoyancy and drag can be harnessed during hydrotherapy to improve gait rehabilitation outcomes. The review identifies research gaps and outlines future opportunities for designing effective underwater robotic systems that complement therapeutic needs.