As an ex waterbird instructor with the Canadian Air Force, I have thrown an S61 at the water about a couple of dozen times for my sins.
From memory:
If the sea state is anything greater than 2-3 then you will overturn very soon after landing. That's assuming you haven't flared too hard and too long and chopped your tail off on the passing swell. This will cause you to rotate rapidly as you bring the collective in (with engines on).
Correct technique is to envisage an invisible touchdown point about 20 feet above sea level (dependent on a/c type) and plan to come to a hover before settling vertically onto the top of and at 45 degrees to...the next swell.
IF you are still able to control the a/c (say u have MGB oil leak etc) then discharge your pax prior to landing and then hover taxi away from them (avoid drift back onto them) before ditching on your lonesome. Reason for this is that WHEN you tip over, you may get trapped but atleast your pax are out
On ditching, if you think you are going to swipe the swell, consider swiping it with the advancing blade first so the equal and opposite newton law rips the MGB out of its housing and propels it backwards away from your seat
If you settle safely onto the water, I would stay inside the a/c strapped in until it rotates and sinks....obviously the doors have gone by now and when the rotors stop turning..wave goodbye to the cab and calmly release your harness and mic tel lead and swim away from the rotor diameter before inflating your jacket.
Landing on water engines off -
Flare, check level - dont worry about tail being choppped.
settle vertically.
Carry on from there.
A BRAVE PILOT WILL ELECT TO GET AIRBORNE AFTER SETTLING ON THE WATER FOR SOME TIME.