Combine all the pork ingredients in a resealable plastic bag and marinate for 30 minutes. Meanwhile, for the salsa, whisk together the rice vinegar, olive oil, soy sauce and sesame oil in a medium bowl. Gently fold in the watermelon, cucumber and avocado. Refrigerate until chilled.
Preheat a grill to medium heat. Remove the pork from the marinade, pat dry and grill for 2 minutes on one side. Turn and grill 1 more minute; remove from the heat and let rest 3 minutes before cutting into 1/2-inch pieces.
Fill the lettuce "shells" with pork, top with salsa and devour!