Prepare a high gas or charcoal grill fire. In a small bowl, combine 1 tsp. of the olive oil with the ground chipotle. Brush the watermelon slices on both sides with the chipotle oil and sprinkle with 1/4 tsp. salt. Grill the watermelon until charred spots appear on both sides, about 1 minute per side. Transfer to a cutting board. When cool enough to handle, remove the rind, seed if necessary, and coarsely chop the flesh.
In a food processor, combine the watermelon flesh, tomato, cucumber, cilantro, shallot, serrano, and the remaining 3 tsp. olive oil. Pulse until the ingredients are mixed but still chunky. Transfer the salsa to a medium bowl and stir in the lime juice. Refrigerate to let the flavors meld, about 1 hour. Just before serving, season to taste with salt.