I was thinking about how she got there also. Here is one theory:
If she were already deceased at the apartment, then maybe she was put in a suitcase and transported out of the apartment and dumped in the river. If her body has been inside the suitcase for awhile, it may have protected it somewhat from the rocks, etc., on the bottom of the river. It may have been the suitcase and not her that was weighted down. So the rocks and other debris may have eventually ripped the suitcase open, allowing her body to float to the top. Oh how I hated writing that...
Because of rigor mortis, LE (well, the ME) should be able to tell if something like this happened. I feel that she either walked or was helped to walk out of the apartment, or her body was concealed some way. I don't think someone carried her body in plain site out of there. Remember MJ was doing laundry and supposedly still moving out. The moving part doesn't make sense to me since DB at one point said she was surprised that he was even there that day.