Use a chef's knife and cutting board to dice the watermelon into bite-sized cubes, aiming for uniform pieces that will hold their shape.
Finely chop the red onion and cilantro; seed and finely chop the jalapeño peppers, making sure to remove the seeds if a milder salsa is desired.
Combine the diced watermelon, chopped red onion, cilantro, and jalapeño in a large mixing bowl, stirring gently to mix the ingredients evenly.
Juice the lime and add the lime juice and zest to the bowl, then sprinkle with salt. Mix thoroughly to coat all ingredients with citrus and salt, allowing flavors to meld for 5-10 minutes.
Taste and adjust seasoning with more lime juice or salt if needed; the salsa should be balanced between sweet, tangy, and salty flavors.
Serve immediately in a bowl garnished with additional cilantro if desired, showcasing the colorful, chunky texture with a juicy appearance.