Place diced pork into a resealable plastic bag and add ancho & Morita sauce. Seal bag and massage to evenly distribute marinade. Allow marinating at least 30 minutes (or overnight for more intense flavor).
Meanwhile, combine watermelon, cilantro, onions, lime juice, and serrano pepper. Toss to combine. Season with salt and pepper. Set aside.
Preheat a non‑stick skillet over medium‑high heat. Add oil and marinated pork, and stir‑fry until pork is crispy about 10 to 12 minutes.
Remove to a bowl and season to taste as needed with salt and pepper.
Build tacos by placing a little pork on each tortilla followed by watermelon pico. Garnish with your favorite taco condiments and serve.
Chef's Note: For street tacos, I like to double stack my tortillas to keep tacos from falling apart.