The reason for using water bags/blocks is safety. If someone fell in the pool when it is covered, they could pull the weights holding the cover down into the pool as they grab the cover struggling to climb out. A weight would sink to the bottom, possibly dragging the cover and person with them. Not good.
A water bag/bottle/block would not sink in water giving someone a chance to climb out, or even just keep their head above water, even if wrapped in the cover.
Don't use weights, bricks, cinder blocks, etc. to hold down a cover. Something filled with water is much safer.