It's a very int eresting question and I would like to know if anyone finds any major statements of the basic stability performance of offshore helos. On the whole I think it is one of those areas where manufacturers do what the aviation regulators require (i.e. not much). The military sounds like a good source - if it is accessible!
If you look at real ditchings, the way that the helicopter enters the water will always make a big difference to the outcome, pretty much regardless of the water stability of the thing when it is turned into a boat.