Save even more time by purchasing precooked shrimp.
Crack eggs into a small bowl. Add water, salt, and pepper; whisk until well blended and frothy (do not overbeat). Set aside.
Heat an 8-inch omelet pan over medium-high heat. Add butter; tilt pan to coat. After butter starts to sizzle and before it has browned, pour egg mixture into the pan. Stir quickly with a fork 8 to 10 seconds, or just until egg begins to thicken. As mixture begins to cook, gently lift the edges of the omelet with a rubber spatula, and tilt the pan so the uncooked portion flows underneath. (Do not overcook. The surface of the omelet should be lightly browned, while the center of the omelet should be soft and slightly runny.)
Sprinkle the cheese, shrimp, scallions, and cilantro on one side of omelet. Fold opposite side over the filling and tilt the skillet to transfer the omelet to a warm serving plate. Top with salsa and garnish, if desired. Serve immediately.