Water only where's 8.35 lbs per gal. It looks like a long bed so what thats a 8' bed? Thats not even 6,000 pounds of water (8' long, 6' wide and 2' high). And all full sized trucks are made to be more like trucks, haul more and tow more then mid sized trucks, so I don't think that filling up the bed with water would be that bad on the suspension. Thats just the math, I also have had experience with this at least once a summer since I was a small kid and ive never heard of it being to much of a load for a truck to handle. Which by the way its amazing last time my buddy had a water jet in his :D
Brandon