Peel and devein the shrimp.
Cut the shrimp open in a butterfly. (You can also chop them up into 1-inch pieces).
Add 1 cup of lime juice and salt to shrimp.
Cover with plastic wrap.
Place in the refrigerator for 30 minutes.
Meanwhile, remove the stems from the serrano peppers. Then, slice them in half.
Remove the seeds from the peppers.
Add the serrano peppers, cilantro, and remaining lime juice to the blender.
Blend until smooth.
Taste for good measure.
Take the shrimp out of the refrigerator.
Add the cucumber and red onion to the shrimp.
Pour the sauce all over the shrimp, cucumber, and onions.
Cover with plastic wrap again and return to the refrigerator.
Marinate for 30 minutes.
Serve with saltines or tostada.
Optional: you can top with avocado slices.