Read a Men's Journal piece out on the Deepwater Horizon, "The Well from Hell." Readers may puzzle at a comment to the effect that offshore workers rank helicopter travel near the top of their fears. I came across this opinion also, when researching the loss of the Ocean Ranger rig off Canada.
The main reason for the workers' worry is that helicopters tend to roll over soon after ditching in an emergency. This can happen very quickly, so each passenger must unbuckle and find his way out of a flooding, crowded, inverted, dark compartment at the same time that other highly motivated people are trying to do the same. Although there is time to do it right (a flipped aircraft doesn't sink like a stone, even when the doors are open) but people who heard a safety briefing beforehand sometimes forget this in the excitement. The challenge is compounded greatly if this happens in a storm in frigid seas because the survivors must also find and don their survival suits, before the helicopter sinks. There are companies that train offshore workers specifically in how to get out of an inverted helicopter in the water.
Why do standard copters tend to roll over in the water? Think of the difference in how weight is distributed in an airplane compared to a helicopter. The heaviest portions of an airplane are the engines, payload, fuel, wings, and the landing gear assembly. In airplanes these are near the center of gravity or below. But a transport helicopter carries some heavy iron above the CG: commonly powerplant, transmission, main rotorhead, and main rotor blades. In short, it's top-heavy.
The most likely rotorcraft to stay upright in the water are ones with permanent flotation gear like Navy Sea King rescue helicopters. But the helicopters the offshore workers are worried about aren't the Sea Kings - they're worried about commercial transport helicopters that roll over after ditching.