After reading this entire thread, here is my theory:
I don't think that Charles Rutherford is alive, and I do think he is responsible for Lana's death.
It's pretty obvious that he was an abuser and came close to killing Lana in the past.
I think that the knob got stuck in Lana's shoe as she was trying to scramble on top of the boat and stomped down on the knob, possibly also slicing the back of her shoe as she slid.
I think that Charles Rutherford was struggling with her, trying to either beat her or throw her overboard, and either she went overboard first (whether due to slipping off the boat or being thrown off by her boyfriend) or he grabbed her and threw himself overboard on purpose while dragging her with him.
If it was the former, in his drunken, enraged, irrational state (a state documented previously by the eyewitness to him beating her in an earlier event) he either stayed on the boat for a while and taunted her while she tried to climb back on, eventually driving the boat away from her and drowning himself later, or else he dove in after her without a life jacket, ladder, or floatation device. -Again, the fact that someone had seen him once drive into a parked car and keep driving in a rage even after his airbag had deployed leads me to believe that after finishing off a lot of vodka and working himself into a rage, he wouldn't be thinking clearly enough once Lana went into the water (no matter how she ended up there) to rescue her properly.
I think he must have drowned not long after regardless of which possible scenario took place, because there just doesn't appear to be a way he could have made it back to shore and disappeared without help, both at the time and later financially, and I just don't see as having been planned ahead of time.