Apart from the drain plug on the bottom right of the outside of the heater, there is a pressure relief valve at the top left of the heater which may have been left in the open position, or the O ring has failed , or the valve has broken and needs replacing.
Other than that there is also a fusible plug in the centre of heater under the outside cover plate.
Take the outside cover plate off ( Phillips screwdriver, 4 screws) and see where the water is coming from.