Puree the mint, vinegar, grape seed oil, agave, 1 teaspoon salt and 1/2 teaspoon pepper in the blender. Set aside. Using a 2- to 3-inch cookie cutter, cut the watermelon into 8 rounds. Slice the radishes thin and set aside.
Pat the scallops dry and remove the “foot.” Sprinkle with salt, pepper, and a little cayenne pepper.
Heat a large skillet to high. Drizzle a scant amount of oil in the skillet and carefully rub it around with a paper towel. When the skillet is smoking hot, place the scallops top down. Sear for 1 1/2 minutes until you can see caramelization around the edges. Using tongs or a metal spatula, flip the scallops. Add 1 tablespoon of butter to the skillet and cook 1-2 more minutes.
To assemble: Place one watermelon round on each plate and sprinkle arugula and radishes around it. Top the watermelon with a large seared scallop. Drizzle with mint vinaigrette and serve while the scallop is warm.