All cases are based on circumstantial evidence.
He was fishing in the bay.
Her body washed up from the bay.
He had a girlfriend who was told "I lost my wife" before he lost her.
Use your common sense.
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?