Place the peeled and butterflied shrimp in a large bowl. Season them with 1/2 teaspoon salt and 1/4 teaspoon ground black pepper. Pour the freshly squeezed lime juice over the shrimp, ensuring they are well coated. Cover the bowl and refrigerate for 25 minutes, allowing the shrimp to 'cook' in the lime juice. The shrimp will turn opaque as they marinate.
While the shrimp marinates, heat a skillet over medium heat. Add the unpeeled garlic cloves, dried chile de arbol, and dried chile piquin. Toast them, stirring frequently, until light char spots appear on the chiles and garlic, about 30-45 seconds. Immediately transfer them out of the skillet to prevent burning.
Peel the toasted garlic cloves. In a molcajete or mortar and pestle, add the coarse rock salt (or sea salt) and peeled garlic. Grind to form a smooth paste. Add the toasted chiles and continue grinding until the chiles break down into flakes, creating a deep-flavored paste. I find that a coarse salt really helps create a smooth paste and enhances the flavors.
Pour half of the lime juice from the marinated shrimp (from Step 1) into the molcajete with the garlic-chile paste. Scrape down the sides to incorporate all the flavors. Stir in the El Yucateco Chile Jalapeño Hot Sauce, Jugo Maggi, soy sauce, Worcestershire sauce, and Clamato. Mix well until you get a smooth, flavorful sauce.
To the bowl with marinated shrimp and the remaining lime juice, add the peeled, seeded, and sliced cucumbers and the thinly sliced red onion. Toss gently to combine and distribute the flavors.
Pour the sauce mixture from the molcajete (from Step 4) into the bowl with the shrimp, cucumber, and onion mixture (from Step 5). Gently fold everything together until well coated. I like to let it sit for a few minutes so all the flavors meld before serving.
Arrange the aguachile negro on a serving platter or individual plates. Garnish with ripe avocado slices and serve with corn tortilla chips on the side for dipping. Enjoy this refreshing and vibrant dish right away for the best flavor and texture.