I regard the “Water arc explosion” phenomenon as a particularly interesting chapter of COP>1 because the energy transformation is between electric to kinetic instead of heat, which lends itself to adoption of practical uses with relatively ease.
Regardless of the controversial aspect of the energy efficiency, water arc explosions have indeed been subject of research for propulsion systems, even for space propulsion applications.
This IEEE paper tries to call attention to the propellant action potential of the water arc phenomenon, and points out the possibility of it being high efficiency.
This is a paper from Tajmar (2000) about a theoretical review of the potential of water arc explosions as electric driven propulsion systems in space:
This is a 2015 report from a Polytechnic school graduate that explored the water arc explosions for marine propulsion, very interesting, they even developed a small scale prototype. They use the original “molecular bond” explanation of Graneau to explain the source of the energy but they reckon there’s not really any consensus about that.