Place tuna steaks in a shallow baking dish. In a medium bowl, mix soy sauce, sherry, vegetable oil, fresh lime juice, and garlic. Pour the soy sauce mixture over the tuna steaks, and turn to coat. Cover, and refrigerate for at least one hour.
Preheat grill for high heat.
Lightly oil grill grate. Place tuna steaks on grill, and discard remaining marinade. Grill for 3 to 6 minutes per side, or to desired doneness.