Use a chef's knife and cutting board to evenly dice the watermelon into small, uniform cubes around 1/2 inch in size, then transfer to a mixing bowl.
If using whole cumin seeds, toast them in a dry skillet over medium heat until golden and fragrant, about 2 minutes. Allow to cool slightly, then grind to a fine powder using a mortar and pestle or spice grinder. Add ground cumin or directly measure ground cumin for convenience.
Squeeze fresh lime juice using a lime juicer until approximately 2 tablespoons are collected, checking for any seeds.
Combine the diced watermelon in the mixing bowl with the lime juice, cumin, chopped cilantro, and salt. Use a spatula or spoon to gently toss all ingredients together until evenly coated and combined. The mixture should look vibrant with visible bits of cilantro and evenly distributed seasoning.
Cover the bowl with plastic wrap and refrigerate for at least 10 minutes to allow flavors to meld. The salsa is ready to serve once chilled and well combined.
Transfer the watermelon salsa to a serving dish, optionally garnish with additional cilantro, and serve cold as a refreshing condiment or appetizer.