Underwater robots have a variety of potential uses, including marine resource research, ecological research, and disaster relief. Most of the underwater robots currently in practical use have screw propulsion systems, which have several noises, collision, and entrainment problems. There is a lot of research on underwater robots using soft actuators to solve these problems. However, current soft actuators have disadvantages, such as the need for special fluids, pressure sources, and high voltage circuits. Therefore, we have developed a soft-skin actuator based on magnetohydrodynamics (MHD). The soft-skin MHD actuator is made of soft material and the structure is prepared as thin, which allows it to attach to the surface of an object, including curved surfaces, to provide the object with a propulsive function in the sea. Since it has no moving parts, it does not generate mechanical noise, and there is no danger of entrapment. Because it can pump seawater directly, it does not require a special working fluid, and its structure is simple and easy to miniaturize. This paper investigates the thrust and power consumption of the developed soft-skin MHD actuator when attached to a flat surface. As a result, we obtained a thrust of 1.37 mN from a single soft-skin MHD actuator with a maximum power of about 140 W. We also measured the thrust force by attaching it to a curved surface. We obtained a higher thrust on a curved surface by adjusting the crossing of the magnetic field and the current than when using a flat surface. We developed an untethered robot that can remove oil from the sea using soft-skin MHD actuators. We demonstrated the adaptability of the soft-skin MHD actuator by attaching it to a commercial underwater camera weighing about 253.5 g and providing propulsion.