Caylee was already dead.........she was going to put her in the pool and act like she accidentally drowned, but somehow realized it would not work. The child was toward the middle of the pool and she didn't want to get wet, so she either stood on the ladder or that box that was there and pulled her body underwater with the shovel until she got her to the side where she could get her out and put her in the trunk of her car.
Makes sense to me because an aluminum pool pole would not have worked for a dead body and she needed something stronger. The shed was locked so she borrowed the neighbor's shovel, then took it right back. I am betting that shovel had decomp and chlorine both on it if that is what happened.