All water containers do is simulate the weight of a fully, or much less (or more) than fully loaded, ride vehicle based on the average guest weight. Because they use water, they can simulate different weight spacings/average weight on different runs and see how it alters the speed of the ride. (More "what happens when the ride has heavy guests in back, and light weight guests, or no guests, in front), it isn't "let's put some water dummies on the ride and see if they survive!"
The fine tuning of ride will need more and water dummies, and will be a lot more precise. Though there would be minimal safety risk to the live human testing, the idea is that some rides, e.g. Indy, have needed to be toned down a bit before they're ready for primetime. There is a difference between getting whiplash, and feeling as though you have gotten whiplash as well as just being uncomfortable. Safe doesn't necessary mean comfortable and no risk of minor injuries during daily operation. You need feedback, and you can't get that from a water dummy.