Simple fact is helicopter as an aircraft is not as safe as a plane.
Plane's engine(s) fail the pilot at least has the ability to glide and control a chance of controlling a crash landing. I would presume part of the annual top up training commercial pilots go through in the simulator is how you crash land in numerous conditions and terrains.
When a plane lands on water the shape and design makes it actually quite stable so in theory it will take alot longer to sink, certainly long enough to get the survivors out and onto life rafts.
Helicopter engine fails and it drops out of the sky. They are also extremely unstable which is one of the reasons riggers do the training in the water tank when the mock helicopter cabin is dropped into the water and then turns upside down. Even if the airbags deployed when it hit the water even in a mild swell it wont take much to capsize and then it comes down to several factors such as how experienced the passengers are, how well they can actually cope with the shock of the initial crash and suddenly being submerged into sea water, that they didnt inflate or accidentally activate their life jacket before they managed to get out through the windows etc.
The Superpuma has been used since the late 70s and given the number of flights they make around the world every day they are about as safe as a helicopter can be. Like all aircraft they go through serious checks before they are allowed to take off and if any fault is found in any aircraft it is not allowed to leave the ground until 1) they fixed the faults and 2) they know what caused it 3) they know how they fixed it. The sad fact is compared to a plane the helicopter is a deathtrap.