no...Lemon juice is an excellent treatment for a loose motion type illness, but not the best.
The best thing is salt water, simple and easy. This is because it not only rehydrates the body but also replenishes Na+ (sodium) ions, which are required for muscle usage and nervous transmission. Losing salt will immediately cause cramps, besides a lot of other problems.
Lemon juice would replenish vital vitamins and other chemicals, but salt comes first.