8 cups 1 1/4-inch chunks seedless watermelon (about 6 pounds)
3 pounds ripe tomatoes (preferably heirloom) in assorted colors, cored, cut into 1 1/4-inch chunks (about 6 cups)
1 teaspoon (or more) fleur de sel or coarse kosher salt
5 tablespoons extra-virgin olive oil, divided
1 1/2 tablespoons red wine vinegar
3 tablespoons chopped assorted fresh herbs (such as dill, basil, and mint)
6 cups fresh arugula leaves or small watercress sprigs
1 cup crumbled feta cheese (about 5 ounces)
1/2 cup sliced almonds, lightly toasted