A submersible pump is a mechanical device featuring a hermetically sealed motor tightly to the pump body, allowing the entire unit to operate completely submerged in the fluid it pumps. Instead of pulling or suctioning liquid from above like a surface pump, it pushes fluid upward to the surface, making it exceptionally efficient for deep-water extraction. 
Because it operates underwater, it eliminates the need for manual priming and is naturally protected against cavitation.