Some thoughts.

1. Over 40 amps equal simulated catastrophic failure.
2. Last flight payload is 13 pounds greater than first flight payload.
3. There are four distinct flight load design problems.

Clearly you have a current/battery management problem. You will need maybe 40% more power for the last flight than the first.

1. Consider a current limiter to be mandatory.
2. Consider limiting the power at each flight load, which would force the pilot to fly on the wing to conserve batteries. A simple method would be to switch additional batteries into service as additional water bottles are added.
3. Consider a prop change at each increase in load. A compromise prop for 15 pound and 30 pound loads is not likely to be found.

Sounds like an interesting project.

Bill