Abu Dawud and At-Tirmidhi narrated that 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)."
There are several Ahadith regarding watermelon none of them are authentic, except the Hadith that we mentioned above.
Green watermelon is cold and wet and is sweet. it also 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."
Taken From: Healing With The Medicine Of The Prophet (peace be upon him) By Imam Ibn Qayyim Al-Jauziyah