Halacha: What Blessing is Recited on Watermelon?

For watermelon, the blessing 'Ha'adama' (the first blessing) is recited.

And after eating, the 'Borei Nefashot' (final blessing) is recited.

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The amount eaten to require a final blessing (from "Daily Halacha" based on the rulings of Maran Rabbi Ovadia Yosef zt"l):

For anything a person eats, they must recite a "first blessing," such as the blessing "Shehakol" or "Haeitz." Even if only a small amount of the food is eaten, a blessing is required, as long as it is eaten with the intention of eating. (However, if one only tastes a food to adjust its seasoning, no blessing is recited for that tasting.) Hence, a person chewing flavored gum, even though only a very small amount is actually swallowed, must bless for the enjoyment of the gum's taste. Likewise, when someone drinks something, a blessing should be recited even if only a single drop is drunk.

This applies specifically to the first blessing, but a final blessing is only recited when a "significant quantity" of the food is consumed. This significant quantity is a "kezayit," which is twenty-seven grams, and for drinks, the measure is a "revi'it," which is eighty-one grams.

Furthermore, eating or drinking should be done within a timeframe that combines the eating or drinking into a single act. If a person were to consume a "kezayit" portion of bread throughout an entire day, bit by bit, they certainly wouldn't be able to recite the Birkat Hamazon afterwards. Instead, the "kezayit" portion must be eaten within the timeframe of "Achilat Pras." There is a dispute among the poskim regarding this time frame: some say it is five minutes, others say approximately seven and a half minutes (and there are other opinions). Therefore, it is correct to always try to eat a "kezayit" portion (especially during Shabbat meals) within four and a half minutes to fulfill all opinions.

In the case of a final blessing on drinks, one must drink a revi'it of the beverage (which is the measure of eighty-one grams) in one go. If sipped intermittently, a final blessing is not recited. Therefore, if one drinks soup with a spoon, no final blessing is recited for that soup, as it wasn't consumed in one go, and likewise, when drinking hot coffee or tea, no final blessing is recited, as it is not possible to drink it in one go.