..In the origins film it was strongly suggested that decapitation is the only way to kill Wolverine, but that Weapon X guy opened his eyes at the end of the credits...also my Wolverine Inside the world of the living weapon book says that it is believed Wolverine would die from decapitation. Does anyone know the answer? Has he been beheaded in any of the universes?

He cant even survive drowning

the Xavier Protocols stated that the only way to kill him was to cut off his head

the drowning thing is relatively new

That's a debatable topic spread out across a few writers. Some instances suggest he can't drown and some can. Most common one is he would drown but his healing would bring him back but then he would just drown again.

As for decapitation, normally it's thought he shouldn't be. But it was stated he was decapitated in WW2 and he came back. It was in Prisoner Number 0. So I think Wolverine volume 3, issue 32 IIRC.