There are many considerations involved in ditching a single (or twin) helicopter at sea following an engine(s) failure. This assumes that the helicopter is not equipped with utility or pop-out floats (a high risk configuration beyond the autorotative distance to land).
Do not forget to open a door(s) or window during the descent and prior to touchdown. A door on a closed cabin helicopter is very difficult to open until pressure is equalized between the outside and inside of the cabin. Should you forget this step you may only be able to open the door when the helicopter is so deep that you or your passengers may find it difficult to swim to the surface.
Use as much rotor energy as possible as you settle into the water. There are debates as to the best way to do this but in my opinion holding full up collective and settling vertically is more effective than an aft cyclic, tail entry method.
Understand sea state and recommended methods for landing in calm or swelling seas. An autorotation into a six foot swell moving at the helicopter creates considerable difficulty for the pilot.
Understand proper methods and use of available instruments and lights to judge flair height. Water is every bit as forgiving as concrete when dropping in from 50 feet.
Use personal flotation devices appropriate to the water temperature and distance from land. Always have them on, not stowed. What you are not wearing will probably not leave the aircraft.
A thorough brief of crew and passengers to include an exit drill or at the very least an exit discussion. This brief must at the very least include methods to open a seatbelt, methods to open a door and when to leave the aircraft (remember the pilot may not be alive or able to give an exit command or instructions).
Use a raft and overwater survival kit. An easily reached breakout knife in case you forgot to open the door is also helpful.