What I gathered by speaking with local pilots points to an engine-out due to a bearing failure, but that's unofficial of course. What appears very likely is that the low flight altitude played a part. Apparently it was relatively common to fly this leg at low altitude, 300ft, until this accident. This was in part dictated by the fact that you fly into the busy Nice ATC (almost never holding, it's a quick suttle-leg that wouldn't be feasible otherwise) where you are required to be below 500ft to stay well clear of commercial fixed wing traffic, which often approaches on nearly the same heading along the coastline). Many questioned this company's pilots (all ex military) practice of regularly flying 300ft at max cruise over the sea. On a loaded A/C, as in this case, there is little time to slow to the required 80kts in order to inflate, inflate and then autorotate to a safe speed (floats do you no good if you impact the water at 60kts) if the single engine quits.
At least one float was recovered on the surface as rescue crafts arrived, which points to too much speed on the water landing. Given the squirrel comes down at about 2400ft min. at full gross (and that's with the engine at idle in training, faster I guess if the donkey has really gone quiet), the pilot had just a few seconds to slam the collective, slow, inflate and flare/stop. This can be done but add a moment of hesitation and/or a tailwind and who knows...
It's all speculation of course, but the fact remains that policy was changed requiring all company pilots to fly this leg at 500ft immediately following the accident.
I am sorry for your sad loss and the fact that I can't provide any more info. And apologies for the late reply, for what it's worth, but I only check this forum occasionally.