On most of CP's coasters, they use bags of steel shot for ballast during test runs. Many of the manufacturers do not approve of this practice for loading rides because while it provides the requisite hundreds of pounds of ballast for the ride, it does not simulate the weight of passengers, because the weight is not distributed well. This can result in damaging loads on various parts of the ride if it is not done carefully.
Of course, on Millennium Force, there is an additional problem: where do you put the weights? Put them on the floor, and there is no guarantee they will stay there. Put them on the seat, and they won't stay there AND you run the risk of overloading (point loading) the plastic seat. The water bodies are a good solution for inanimate test dummies because they more closely resemble the weight distribution of a human, which is what the seats were designed for...and they even have a center of mass not far removed from a human center of mass.
Of course, the problem is that the water bodies need to be securely tied down. One of those things flying through the air could cause real problems!
In the absence of water bodies, of course, human bodies make great ballast for coaster testing. Prior to 1996, that was what Cedar Point used most of the time for daily tests.
--Dave Althoff, Jr.