In a large, non-reactive bowl, blend the soy sauce, olive oil, water, garlic, and pepper. Place the beef tri tip in the marinade. Cover, and marinate in the refrigerator at least 4 hours.
Preheat an outdoor grill for high heat, and lightly oil grate.
Grill the beef slices 3 to 5 minutes per side, or to desired doneness. Discard remaining marinade.