It's not a sin against the muslim deceased but is only to be used when the person dies at sea and is too far away from land to be buried in the earth. However, I think it was appropriate given that the U.S. supposedly said they didn't think anyone would accept the body and that his grave would become a shrine. Although, that might have been a good way of finding terrorists.
I had heard ( in addition to the above ) that burial at sea was also appropriate if there were a more than reasonable chance that the land grave would be disturbed.
In any case there was no reason at all to give him anything "according to Islam". In doing so you have sanctioned his actions as being according to Islam instead of "radical Islam". The "radical" Islams are going to be pissed no matter what.
His body should have been desecrated beyond belief as was Sadaam's.