Have crossed the channel and the Irish sea in a single engined helicopter.
Life jackets, floatation gear on the heli, PLB. Always talking to someone on the RT.
I would expect to be able to ditch the heli, inflating floats before contact with the water, but given the position of the engine & gearbox I would expect the machine to turn over in anything but the calmest of water.
Usually I try to navigate shortest crossing, and where possible I head from "ship to ship". There is some chance that I could put the machine onto a tanker / container ship if needed (there's always a chance!) but if not then ditching nearby (I hope) increases the chance of a quick pick-up. There is no possibility of gliding any serious distance in a heli once the donky dies.
Without a PLB or a raft, sea-dye or similar I think the chances of being seen by a search aircraft might be low indeed! Staying with a floating heli - even one that has overturned - should increase the chance of being seen.