Use a chef's knife and cutting board to cut the watermelon flesh into roughly 1-inch cubes, then transfer them to a large mixing bowl.
4 cups watermelon flesh
Thinly slice the cucumber and red onion using the chef's knife, and add them to the bowl with the watermelon.
1 cucumber cucumber, 0.5 red onion red onion
Juice the limes using a citrus reamer or juicer until you have about 1/4 cup of lime juice, then pour it over the salad mixture.
2 limes lime juice
Add the chopped cilantro and sprinkle the sea salt over the mixture. Toss everything together gently with a spoon until well combined.
1 cup chopped cilantro, 1 teaspoon sea salt
Cover the bowl with plastic wrap and refrigerate for at least 15 minutes to allow flavors to meld and the salad to chill.
Serve the salad cold, garnished with additional cilantro if desired, for a bright and juicy dish.