Rinse the shrimp and pat dry with a paper towel. Heat the olive oil in a medium pan and sauté the shrimp for 1 minute on each side until pink and cooked through.
Add the shrimp, lemon juice, lime juice, and Clamato tomato juice to a medium bowl. Cover with plastic wrap and refrigerate for at least 30 minutes and up to 8 hours.
While the shrimp is marinating, prepare the rest of the ingredients. Remove the watery cores from the tomatoes and dice the skin. Dice the onion and jalapeno (if using), and finely chop the cilantro. Mix all of the vegetables in a bowl large enough to fit the shrimp.
Stir the shrimp into the vegetables. Use a butter knife to cut the avocado into cubes and add it to the bowl. Season with salt to taste.