Very pleasing to see so much cautious advice on this item. Having started my flying career in Navy ASW/SAR role I know how difficult 'night hover over the sea' really is. I would not risk my aircraft/crew trying to do it without radalt height hold and doppler. Basically you do need the kit and regular practice if you do not want to end up in the sea yourself!
I believe many civilian operators work on a 90 day currency plan but that may only be fine with crews who are very experienced.