Prepare a high charcoal or gas grill fire. In a small bowl, combine 1 Tbs. of the oil with the chipotle powder. Brush the watermelon slices with the chipotle oil and sprinkle with 1/2 tsp. salt. Grill the watermelon until nicely charred 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 blender, combine the watermelon flesh, tomato, cucumber, cilantro, shallot, serrano chile, and the remaining 1 Tbs. olive oil. Blend until almost smooth. Transfer to a medium bowl and stir in 1-1/2 Tbs. of the lime juice, the red wine vinegar, and season to taste with salt. Refrigerate until thoroughly chilled, about 4 hours.
When ready to serve, combine the crème fraîche with the remaining 2 Tbs. lime juice in a small bowl. Taste the soup and correct the seasoning, if necessary. Ladle the soup into cups and garnish with a drizzle of the lime-crème fraîche mixture.