This paper presents a novel actuation method to change the buoyancy of an underwater soft robot by pumping denser and less dense liquids, than the liquid the robot is immersed in, into the robot to actively change the mass of the robot and cause it to experience a change in buoyancy. The technological research gap lies in the method of pumping lighter and heavier fluids into a soft robot to cause it to experience a change in mass and depth, which has not been explored before to the best of the author's knowledge. An analysis of the forces that are placed on the robotic system and the necessary equations to determine the force produced by a solution with a particular ratio of solute to solvent are presented. Preliminary experiments were conducted to test the buoyancy-based actuation method discussed in this paper by building a two link, soft, inflatable robot arm. This robot was shown to change the floatation height of its links when denser fluid was pumped into its links.