You will need to make sure your oil of choice has anti-foaming properties. I always go a step above the base cheap bucket of oil. Look for an oil labeled "tractor hydraulic" or "transmission hydraulic" these should be your better grade of oil.
Water in a hydraulic system especially one with hydrostatic drive is very,very bad. There is no good way to get the contaminated oil out of the system, it will be expensive but I would change it several times.