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Well, water weighs a little over 8.6 pounds per gallon so it seems like a good way to add weight for the winter and it appears to have internal baffeling to prevent sloshing so that's a good thing. The downside is that it renders your bed unusable for the entire winter. No matter how tough they claim it is I doubt you can toss a tool box (or anything else) in there on top of it. Also, I wouldn't want to park on an incline when it's going to get below freezing at night or you'll wind up with a very off center weight in the bed. Other than that it seems like a good idea.
Personally, I toss a few of bags of marble chips from Home Depot in the bed in the fall and in the spring I use them in my garden.