A self-powered self-contained electrolysis device, for placement into a reservoir of a contaminated electrolytic solution, such as water, containing halide ion, such as chloride ion, to electrolyze the water, thereby disinfecting or sterilizing the contaminated reservoir of water. Contaminated reservoirs of water can be water containers filled with river water and other outdoor sources, or can be contaminated municipal water held in kitchen containers, cooling systems, water tanks, cisterns, etc. The self-contained body allows the electrolysis device to float on or remain self-contained in the reservoir water. Preferred devices are small and portable, and comprise reliably productive electrolysis cells that are powered by batteries. A means for propulsion of the device can also be provided, and is preferably a pump that pumps the water through the electrolysis cell.