RAD ALT is the absolute last resort. If that saves your bacon you have already seriously "Fuc?ed up".
I have to disagree - a rad alt with audio and visual warnings and a good set of standard operating procedures is a template for safe operation over water, day or night.
It is the same as using it for NVD low level ops, if the light or audio gives an alert then you react appropriately and climb until it clears.
You just have to be disciplined in setting it correctly and appropriately.