Watermelon is not considered to be citrus fruit because of where it falls on the pH scale. The pH scale was designed to measure acidity, in this case for fruits. Some examples of citrus fruits include oranges, lemons, limes, and grapefruit.
Those who have problems with acid reflux will definitely want to avoid eating fruits like oranges and lemons because of the high level of citric acid in them. These fruits have been known to make acid reflux or heartburn worse, so if you have this problem you will want to limit the amount you consume in your daily diet.