Watermelon was the most beloved fruit to the Prophet (peace and blessings be upon him)

It is narrated from Sayyiduna Anas bin Malik (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) would hold fresh dates in his right hand and a melon in his left, then he (peace and blessings be upon him) would eat the fresh dates together with the melon, and it was the most beloved fruit to him (peace and blessings be upon him).
Yusuf bin Atiyyah is alone in narrating this hadith, and the two Shaykhs (Bukhari and Muslim) did not use him as evidence. This text is also known, in wording other than this, from the hadith of Sayyidah Aishah (may Allah be pleased with her).

It is narrated from Sayyidah Aishah (may Allah be pleased with her) that the Messenger of Allah (peace and blessings be upon him) said: "Eat unripe dates together with ripe (dry) dates, for when the son of Adam eats that, Satan becomes angry and says: The son of Adam has remained (alive so long) that he now eats the new together with the old."

It is narrated from Sayyiduna al-Miqdam bin Ma'dikarib (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "The son of Adam has not filled any vessel worse than his stomach. A few morsels are enough for a person to keep his back upright. But if (eating more) is unavoidable, then one third should be for his food, one third for his drink and one third (left empty) for his breathing."