Food combining (also known as trophology) is a term for a nutritional approach that advocates specific combinations of foods as central to good health and weight loss (such as not mixing carbohydrate-rich foods and protein-rich foods in the same meal). One randomized controlled trial of food combining has been performed, and found no evidence that food combining was any more effective than a "balanced" diet in promoting weight loss.
Food combining adherents believe that watermelon and other melons should be consumed separately from other foods. 
^Golay A, Allaz A, Ybarra J, Bianchi P, Saraiva S, Mensi N, Gomis R, de Tonnac N (2000). "Similar weight loss with low-energy food combining or balanced diets". Int. J. Obes. Relat. Metab. Disord.24 (4): 492–496. doi:10.1038/sj.ijo.0801185. PMID10805507.