We don't know why they're dead. Lack of information. Coulda, woulda, shoulda, speculation.

Did they lose consciousness prior to impact? Fuel leak in the cockpit? Fire? Stall it above the surface? We don't know. Impact a wave? We don't know.

The Twin Otter lands at roughly the same velocity as a sparrow, but it's got tall water-grabbing gear. For those who don't land on water or who haven't, there are a wild ideas about how to do it, and if the surface, or the angle to approach the surface, makes depth perception difficult (glassy water landing, or lighting that precludes depth perception), even birds sometimes can't find the surface and fly into it or stop flying above it. Just a fact of landing on water. Was it a factor? Unknown. I've run into a lot of pilots who suggest that they'd "stall it on" to minimize impact forces, completely oblivious to the fact that they'll nearly certainly make the landing worse.

Waves can be hard to gauge. Again, if someone hasn't landed on water, it's a moving runway, horizontally (often diagonally) and vertically.

The success of ditching also depends on whether either pilot was able (awake, conscious), and that information is also not in evidence. Radio calls were made while descending, but their condition at or just prior to impact is not given. Autopsy will provide that.