Even my common sense can't account for one thing in this case: what's with the cement anchor? If he made a few, for the purpose of sinking a body, why go home with one? Why not drop them all?
maybe just in case amber broke up with him he'd have one handy.
You got me! Maybe we should try to think like a killer!
He got careless? Wanted to go back (in the boat) later and check the location of the bodies?