Tomato-Watermelon Salad with Feta and Toasted Almonds

8 cups 1¼-inch chunks seedless watermelon (about 6 pounds)

3 pounds ripe tomatoes (preferably heirloom) in assorted colors, cored, cut into 1¼-inch chunks (about 6 cups)

1 teaspoon (or more) fleur de sel or coarse kosher salt

5 tablespoons extra-virgin olive oil, divided

1½ 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)

½ cup sliced almonds, lightly toasted