Maybe it was one of his couriers or hangers-on who was also shot at the Abbottabad compound. I can see lots of public relations reasons why it's useful to appear to bury bin Laden while actually keeping his body frozen for later forensic examination.
The great part of this conspiracy is there is no way to prove it false. Do you suppose official chain of custody (CoC) rules were applied to putting him in the water? Highly unlikely.