This study proposes operating a Hall effect Thruster (HET) on the products of in-situ water electrolysis. Oxygen is supplied to the anode and hydrogen to the cathode to mitigating poisoning. In comparison to other alternative HET propellants water is non-toxic, low cost, highly storable, and allows synergies with chemical propulsion systems. The competitiveness of this concept is experimentally tested by simulating, designing, constructing, and operating a thruster optimised for oxygen: the Water ElecTrolysis Hall Effect Thruster (WET-HET). A zero-dimensional particle-in-cell code is used to optimise the device, resulting in a channel width of 5 mm, outer circumference of 25 mm and depth ranging from 35 mm to 60 mm. Iterative magnetic, thermal, and mechanical modelling finalises the design. A hanging pendulum style thrust balance is used to directly measure WET-HET performance. Safety restrictions require krypton rather than hydrogen be used for the cathode. The WET-HET is tested on discharge powers up to 3200 W and oxygen mass flow rates ranging from 0.99 mg/s to 1.85 mg/s. Three different channel depths are tested: 35 mm, 45 mm and 60 mm. The WET-HET is characterised for a range of different magnetic field strengths and for three distinct magnetic topologies. Two channel wall materials were investigated: alumina and boron nitride. The highest thrust measured was 38.63±0.25 mN, with a maximum specific impulse of 4112±36 s and a maximum anode thrust efficiency of 24.4±5.9%. This optimum was found when the WET-HET was operated at 3200 W discharge power, 0.99 mg/s mass flow rate, 35 mm channel depth and a magnetic field that peaked near 600 Gauss. Boron nitride outperformed alumina by approximately 40%. Contrary to expectation, increasing the axial thickness of the magnetic region had little impact on discharge voltage, but led to a reduction in thrust, specific impulse, and thrust efficiency of the device.