Define what you mean by "his own action." (Just so we don't go back and forth, you haven't determined what the body of water is.)
Did he trip and fall into the water. (Sorry, I'm trying haha)
No.
I think I see what Foote is getting at.
Was Wayne's death a suicide attempt?
If not, was Wayne his own murderer?
If not, was there a human agent that murdered Wayne?
Are you sure it wasn't Garth?
Was the water flowing?
No. Yes and no. Yes.
No. 
Yes.
Is the water outside?
Yes.
Was it a river?
Yes.
Did he slip on a rock?
Was Wayne intoxicated, perhaps from too much partying on?
No.
No.
Was anyone besides Wayne involved in his death?
Was Wayne's murder premeditated?
Had Wayne met the murderer previously, i.e. on some other occasion before the murder?
Yes. Yes.