To prepare a watermelon injected grilled pork fillet, you'll first need to prepare the marinade. To do this, mix together 1 cup water, 1 tbsp. salt, and 12 c. kosher salt. Add the pork and marinate for at least 1 hour, or until the meat is no longer pink in the middle and the juices run clear. Then, remove it from the refrigerator and allow it to rest for 30 minutes before slicing it into 1/4-inch-thick slices. Place the slices on skewers and grill them over medium-high heat until they're charred on all sides, about 5 minutes per side. Serve with guacamole and salsa.