Let's take this seriously for a minute.
The Lynton & Lynmouth Cliff railway works thus:
The car at top of cliff fills watertank till full:
When top & bottom cars are ready to go bell codes are exchanged:
The operator on car at bottom discharges water till the weight of the top car over comes the weight of bottom car and lifts the now lighter Bottom car:
Now if 20 Passengers are taking 20 X 2 gallon buckets up with them then to overcome the extra weight of the 40 gallons of water in the bottom car, then the bottom car will need to discharge an extra 40 gallons of water to overcome the extra weight:
Total water saved is ZERO