Let’s say your home, work, shopping, and entertainment destinations are all located at some point along a more or less straight sloping road. Let’s say the slope is a generally monotonic 10% grade more or less. The distance between destinations is not so important, but let’s say between 5 and 50 kilometers. Let’s say you put water bags in your tesla and use them as follows: When driving uphill, the water bags are empty. When driving downhill, the water bags are full, and you drive slowly to avoid aerodynamic drag. The slope of the hill is such that you have to keep your foot on the brake at all times when driving downhill. Assume your battery charge level is around 50% so the battery can accept charge from the regenerative braking. How many pounds of water must you carry downhill to overcome overall systemic roundtrip uphill and downhill losses?

A lot. Without accounting for the energy spent driving the horizontal distance, your water bags have to make up a percentage of weight that matches regeneration efficiency just to balance the potential energy. So if you assume the car is 70% efficient on the total round trip (two passes through the inverter/motor losses plus the battery loss from a charge/discharge cycle,) then the car has to be 70% of the weight going down - you need water bags that weigh almost half as much as the car, then some extra to carry the car along the horizontal distance.

To be helpful, the water would need to be free or very cheap. For example locally captured rainwater. Not pumped. No need to be potable.

Most of the energy loss is due to i-squared-r losses on the uphill leg. Regen recovery is usually above 95% unless braking activity is significant. 1000 pounds of water would likely do the trick, but the easier solution would be to stay under 25 mph on the steep grades, which would work almost as well.

just quick back of the napkin math you'd need about 1247.6 lbs of water more when you account for passengers and cargo figure 1500 to be on the safe side but when your running against the suggested weight limit and not accounting for weather