Well everything is possible...but drowning? I can see somebody drowning, but why stage it as a crime? There is no need for that...you will be swapped away with the tied, nobody has to know you were even near her when this happened...strip naked and put a bag over her head, you could do that on the scene, again why, but going back home to fetch the weights...(or they must have been in a car...) I don't know. IMO It's sounds and looks like a murder to me and she needs to be identified, whatever the cause of death was.