The interior baffles are supposed to reduce the sloshing, but in my experience, there is still a certain amount of surging of whatever is inside the tank. On one railroad I worked on, we handled tank cars of cooking oils, and at the place where we would set out the loads, when we stopped, we had to set the air brakes before uncoupling, or the sloshing would move the cars enough to recouple them. In fact, even while under way, the surging would occasionally give you a kick in the pants.
While I think keeping water in the car for fire protection is a good idea, the water would have to be treated with some sort of biocide to prevent the growth of algae which could clog the outlet and/or any pump. And the car should be parked somewhere accessible without having to switch out other equipment before taking it to where it is needed most.