Slice the bread 1/4 to 1/2 inch thick and remove the crust from all but 4 slices. Tear the crustless slices into pieces and pulse in a food processor to make coarse crumbs. Add the chopped watermelon to the food processor along with the chopped scallion, tomato, half each of the cucumber and jalapeno and 1/4 cup cold water; puree. With the machine running, gradually add 1/3 cup olive oil and the juice of 1 lime. Stir in 2 tablespoons mint, 1 teaspoon salt and a few grinds of pepper. Cover and refrigerate until ready to serve.
Heat a grill pan over medium-high heat and brush with olive oil. Toss the shrimp and whole scallions with the juice of the remaining lime and 2 tablespoons each olive oil and mint, the garlic, 1/4 teaspoon salt and a few grinds of pepper. Grill the shrimp until marked, about 3 minutes. Flip and continue grilling until cooked through, about 2 more minutes. Grill the scallions until lightly charred, 1 minute per side. Brush the remaining bread slices with olive oil and grill until toasted, 1 to 2 minutes per side.
Top each serving of gazpacho with the diced watermelon and the remaining cucumber and jalapeno; drizzle with olive oil. Serve with the grilled shrimp, scallions, bread and lime wedges.