RAF CFS teaches that the downwash causes a "bowl" shape in grass resulting in increased recirculation. This increases the induced airflow and therefore reduces angle of attack for a given blade pitch angle (similar for water surface). Also, the taller the grass then the higher the aircraft actually is from the true surface (I've seen 8' grass!) and there's less ground effect to be had anyway. The more practical problems are that if you hover too low over long grass then you are likely to "ding" a tail rotor blade. If too low over water the practical problem is self-induced IMC due to spray. The best answer is not to hover over either if the aircraft is so power limited as to be badly affected!