Zest two of the three tangerines. Reserve the zest.
With a sharp paring knife cut away the peel from all three tangerines. Cut out segments from the tangerines, slicing around the white pith. Reserve the segments.
After cutting out the segments, part of the center of the tangerines will be leftover. Squeeze these leftover pieces of the tangerines to obtain four teaspoons of juice. Reserve the juice.
Heat the olive oil in a large skillet until it starts to smoke. Season the scallops with salt, pepper, and cayenne. Sauté the scallops on very high heat, very quickly. Remove the scallops to a large bowl.
Add the tangerine segments, watercress, and onion.
In a separate bowl combine the tangerine zest, tangerine juice, agave syrup, Dijon mustard, and some salt and pepper. Slowly whisk in the 4 tablespoons of extra virgin olive oil until incorporated. Drizzle over the salad and serve.
*Blue agave is the plant from which tequila is made. Whadda ya know. Tequila's a vegetable too! In addition to being sweet, blue agave syrup is quite tasty. If you can't find it in your local supermarket substitute sugar in the dressing or omit the sweetener altogether if you prefer.