In Tibb (Islamic Medicine), watermelon is classified as cold and moist: food qualities.
Ibn Qayyim in "Medicine of the Prophet" said, "The Prophet, peace be upon him, used to eat watermelon with ripe dates, saying: "The hotness of this substance [dates] neutralizes the coolness of that [watermelon]."
Ibn Qayyim noted that, “There are several Ahadeeth regarding watermelon none of them are authentic, except the Hadith that we mentioned above.” And, “Green watermelon is cold and wet and is sweet. It works as a cleanser (for the stomach and the intestines). Watermelon departs the stomach more quickly than cucumber and quickly blends with whatever substances are present in the stomach. It is beneficial to eat watermelon when it is warm, but when it is cold it is better to have it with some ginger to offset its harm. Watermelon should be eaten before the meals. Otherwise, it will cause nausea. Some doctors said that when watermelon is, "Eaten before the meal, it cleanses the stomach and removes ailments."
Ibn Qayyim said, “Eating watermelon seeds with sugar will cleanse the stomach from stones and will prevent burning sensation while urinating."
As-Suyuti in "Medicine of the Prophet" said, "The green variety is cold and moist: the yellow is inclined to be hot. The greater the sweetness of a melon the greater the heat. It is also a diuretic.
As-Suyuti said that watermelon, “cleanses the belly, augments the spinal fluid, aids sexual intercourse, purifies vision, and cuts a cold. But the probability is that these properties reside only in the yellow melon. It is essential that melon be not eaten in times of dire hunger.”