Excellent swimmers, such as tuna, rays, and goldfish, take advantage of their flexible fins, compliant bodies, and swimming bladders to achieve fast, highly maneuverable, and energy-efficient locomotion. Ionic polymer-metal composites (IPMCs) present attractive opportunities for implementation in flexible underwater propulsion systems due to their intrinsic compliancy and underwater actuation capability. IPMCs can also perform as lightweight and compact catalysts for water electrolysis, which can be used to generate gas for buoyancy control. In this chapter, the potential of IPMCs in underwater propulsion is explored, including caudal fin propulsion, pectoral fin propulsion, and buoyancy control. Enabling technologies, including fabrication methods, modeling and control strategies, and design approaches, are developed for creating bio-inspired robots using IPMC as artificial muscle and buoyancy engine. Three types of underwater robots have been developed to evaluate their performance. First, a robotic fish propelled by an IPMC caudal fin is developed to evaluate its caudal fin propulsion. Second, a bio-inspired robotic cownose ray propelled by two IPMC actuated pectoral fins is demonstrated to evaluate its pectoral fin propulsion. Third, a buoyancy control device enabled by IPMC-enhanced electrolysis is developed to explore its buoyancy control performance.