Lab Guys in Canada
We were successful without technology due to the way we used the crew. Overwater hovering can be done with one crewmember looking down telling you where you are in relation to the object in the water. This has an advatange over the floating flares and doppler because the downwash, winds, tide will move the object. What the pilot needs is a horizon and a compass. Maintain the heading and fly an attitude. Adjust the attitude to correct the drift that the crewmember called. I've tried it with just an AI, not really pretty but possible. NVG are good at giving a horizon, previously we had a fixed wing SAR aircraft drop para flares about 2-5 miles away...created a visual horizon.
The question was for currency. I went into the details above for a reason. We did night ship hoisting once per year fur minimum currency (that's a little different because then you have the ship as a hover reference). We didn't have over water night hoisting as a training currency requirement. This worked because the methods I mentioned above were used in every sequence we did. Confined areas, land hoisting, small pads, mountain landings, slinging, ship hoisting (day and night) and waterwork (hoisting, entries, landing) all used precisely the same terminology, same crew duties, etc. A small exposure to the night environment with rigorous daylight training was proven sufficient to maintain a night operational capability.
That being said, many sought more training than what was required. Depending on experience I think monthly sequences in the daytime and quarterly sequences at night should be okay. We normally ended up doing weekly or better daytime sequences and quarterly night time.
The theory behind all this was to not practice bleeding. If you know what to do and won't be surprised by the changed environment, then daytime training will prepare you for the more dangerous night time missions.