I had the opportunity to try my new Flashpoint (Adorama) Water Bottle/Sandbag tonight ($8.50 at Adorama or Amazon). Just wanted to warn everyone not to buy this.
The only good thing about this was the price. However I was a little disappointed from the get-go when I discovered that they are just regular sandbags with a divider that separates them into two water-bottle sized compartments per side.
You could just take a regular sandbag and load it with water bottles. You'd have the flexibility to load it with odd sized objects as well, rather than being forced to the water-bottle dimensions.
The top closes with velcro. These can totally not be used for sand!
Wanna know the kicker? I loaded one up with 4 soda cans, and when I lifted it up by one side, the rivet came out. This caused the fork of the Y to open up. Now it's so wide across that it won't really sit on my light stand well. Also makes the rivet hard to use for hanging (as seen in the product picture). Please see my product photo.