This is what I think happened, instead of drowning. If she drowned, I can't imagine Casey not at least screaming so the neighbors could hear, if not calling 911. That would be a true accident.
Now...if she drugged her kid and left her anywhere, even in an apartment, George and Cindy's house, so she could go out partying, and accidently OD'd her, THAT would be murder. That would be a good reason to hide the body...toxicology would show that Caylee had a lethal amount of Xanax in her blood stream.
I think Casey left Caylee unattended at George and cindy's house to go party, and used the xanax to drug her so she would sleep while Casey was gone.
Then again, if she died at George and cindy's, the cadavar dogs would have hit IN the house, not just outside. So maybe Casey left her somewhere else...backseat of the car, etc.