Prepare the watermelon, tomato, onion, jalapeño, and cilantro as listed above.
In a bowl, mix all ingredients gently together with the lime juice, sugar and salt. Serve immediately or wait 5 minutes to let the flavors meld. Taste and add more sugar or salt to adjust the flavor as desired. Serve with chips or with grilled fish, shrimp or chicken, using a slotted spoon to drain juices before serving. Tastes best the day it is made, but it stores up to 2 days refrigerated (it will gather liquid during refrigeration so you may need to drain or use a slotted spoon for serving).