Although I, like everyone, would want proof, I understand the reasons for burial at sea. First, from what I understand, it’s in line with Islamic rites that the body be disposed of as quickly as possible, second, it deprives his followers from any grave site to turn into some sort of shrine/holy land.
He’s gone and ‘erased’ from the earth. I can’t recall what happened with Hitler’s body, but I would imagine that the same theory was used…deprive his followers from any sort of ‘new mecca’ or new holy land for them to focus upon. Deprive them of a potential shrine and you deprive them of any focus point for following.