Rotorspeed - as I said, the SAR letdowns can be conducted in zero vis/fog/in cloud, however you want to define it - that is the whole point of having a fully automatic letdown capability in a SAR helicopter.
For those FW orientated, think of it as a Cat III autoland that you can do anywhere over the sea providing you are using the radar correctly.
As to min distance from the land - we used to use about 0.5nm as an absolute minimum for the let down and we could close to around 75m in the coupled hover from there.
Although FAA rules permit otherwise, you are not VMC/VFR over the water at night even when you are completely clear of cloud.