As a guide, on a government funded full time SAR unit using a fully coupled autohover machine and NVG, one night session of wets or decks per month would be considered a minimum.
As for night overwater SAR without autohover - tried it - scared myself - don't want to do it again. Risk versus Gain way the wrong way.
The current trend of LIMSAR (which I am told is only Limited in scope and not in depth of capability) is a bit worrying. There seems to be more and more places that are trying to put in place the minimum possible SAR assets to satisfy a Safety Case. Often organisations are 'selling' a capability that the poor sods at the sharp end simply cannot safely deliver.
Good Luck!