To make the salad, toss the watermelon, black olives, red onion, and arugula together in a bowl. Put it in the fridge to chill. The contrast of a cold salad with the hot salmon is really refreshing.
To make the vinaigrette, toast the spices in a dry skillet over low heat for 1 minute, until they smell fragrant; shake the pan frequently to prevent scorching. In a spice mill or clean coffee grinder, grind the toasted spices with the sugar. Put the sherry vinegar and oil in a blender and add the ground spice mixture; give it all a whirl to blend. Season with salt and pepper.
Preheat a gas or charcoal grill and get it very hot. Rub both sides of the salmon with oil; then season with a generous amount of salt and pepper. Take a few paper towels and fold them several times to make a thick square. Blot a small amount of oil on the paper towel. Then carefully and quickly wipe the hot grates of the grill. This will create a nonstick grilling surface. Grill the salmon, skin side down, for 5 minutes. Carefully turn the fillets over and grill the other side for another 5 minutes, until the fish is opaque.
Toss half the vinaigrette with the salad; wait until right before serving to prevent the arugula from wilting. Serve the grilled salmon on top of the salad family style. Easy McEasy.
Edited: October 14th, 2009