Single engine across water? Not sure whether you wanted opinions on doing it, or which heli.
Wear a life-jacket
Cross at the shortest point
Fly high
Fly from ship to ship (within reason)
Go for it!
Despite the inevitable 'auto-rough' the moment you cross the coast, the engine doesn't know it's over water.
Heli type?
The one with floats!
I'd say the one you feel safest in over land. I leave it to the professionals to comment on their 'ditching' characteristics!
Enjoy your flight.
BTW, if you go to Le Touquet, the restaurant at the Perard fishmonger is superb. It looks scruffy, but don't be put off.