Pressure washers are great tools for cleaning stucco and the cool thing about them is the fact that they could be used for so many other things, so ...
To create holes, use a brace and bit to drill through the flooring, and then a keyhole saw to widen the entrance. Make sure that the holes are not exactly above a floor joist when you drill them. In fact, the joists & studying must be spared as much as possible while installing the water heater equipment.