The RAF SARF operate routinely offshore with NVG in the low level (1000' and below) environment. Like any light amplifying device their performance varies with conditions and as you suggest performance on moonless overcast nights can be poor. That said they almost always enhance the capability and safety of operations. In good conditions approaches to vessels and cliffs can be performed with sole reference to NVG, although the automatic systems such as Rad-Alt height holds are still used to reduce cockpit workload. In poorer conditions approaches may be carried out using the ac's radar and full use of the ac automatic systems, but NVG will still be used and often aid in spotting the target earlier or at least aiding SA. Of course in fog they are no use what so ever...!
