Blot scallops dry on paper towels. Season with salt and pepper.

In a large sauté pan, heat olive oil over medium-high heat. When oil begins to shimmer, add scallops, and let sit undisturbed for exactly 1 minute. Reduce heat to medium, turn scallops over, and let sit undisturbed for 1 minute. Remove from heat and add butter. Let butter melt, stirring to coat scallops in melted butter. Remove scallops from pan and set aside.

Toss watercress with just enough vinaigrette to lightly dress. Divide watercress evenly among crackers. Top each cracker with a scallop. Lightly drizzle scallops with remaining vinaigrette.

Garnish each canapé with lemon zest and lime zest, if desired.

Serve immediately.