The idea that you think the water will excrete through urine is definitely wrong.
Because the body has its own homeostasis to maintain. The water lost will be replenish from food and water intake. The one going out is extra water only. Her body still maintain similar level of water before and after this process. On the other hand, once you drink less water, the body will reabsorb water from urine, however, the formed urine still needs to be excreted. This means, you drink less, probably excrete more.
Second, my idea is that the sugar in the watermelon inhibited the activation of lipases in the body that could convert fat into fatty acid and glycerol. I didn't mean that the sugar is sufficient to support her daily allowance of carbohydrate.
Third, have you heard of the weight controlling process for boxers and sportsmen before game? Before weighing, they have to control everything they eat and drink, including water. Go and ask those people. You'll get more idea.
Last but not least, I am studying life science, which I think I have enough relative knowledge to answer this question.
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