Butterfly the sardines, leaving them joining at the back and pulling out the bone with your fingers. In a shallow bowl large enough to hold the sardines, combine the water, vinegar, garlic, oregano, bay leaves, salt, and pepper. Place the sardines skin side up in this mixture and marinate for several hours, refrigerated. Drain the sardines well on paper towels and fold them back together. Dust with flour. Heat the oil at least 1/2-inch deep to about 380 degrees F. Or, better, use a deep-fryer. Coat the sardines with the beaten egg and place directly in the hot oil. Fry until golden, drain, and serve.
Variation: Sardines are also delicious without a marinade. Simply coat with flour (not necessary to remove bones) and fry, or eliminate the flour and grill them. Sprinkle with coarse salt.