It's been done more than a few times. Probably a few GOM guys can spill on it. I know of at least one 206 with an N1 tach failure that successfully auto'd into the ocean. Another time is was a 206L that shut its engine off in the more dramatic flare than usual to a floating pad (eng overspeed?). Everyone in both got out OK, they don't sink that fast. From a pilot's point of view, it is just a normal auto, as the belly sinks in the water put the cyclic to the right so the leading blade hits the water first, retreating blade then doesn't cut into the cockpit and helps pull the transmission backwards instead of forwards. Opposite for the Eurocopters.
A good auto with a slow water entry and keeping it upright is important. There was a 212 that did a bad night auto in the Maldives a few years back and everybody died on the impact with the water, not drowning or hypothermia.
I don't think any of the S-70 doing night ocean SAR have flotation. At least they didn't in "The Perfect Storm". Probably a few of the tuna boat robbies don't have anything either.
Malabo