Mexican Shrimp Aguachile

1 pound sashimi-grade head-on shrimp or 12 medium shrimp, shells and heads removed and reserved for another use (see notes)
Sea or kosher salt
2 serrano chiles, stemmed and cut into ¼-inch pieces
2 tablespoons water
4 tablespoons fresh juice from 4 limes
Freshly ground black pepper
¼ cup thinly sliced red onion
2 tablespoons diced & seeded cucumber
Tostadas and sliced avocado, for serving

Split shrimp in half lengthwise and discard any veins. Transfer to a plate, sprinkle all over with salt, cover with plastic, and refrigerate at least 1 hour and up to 2 hours.
Meanwhile, in a mortar and pestle, combine chilies with a pinch of salt and pulverize thoroughly, then add water and continue to work until chilies are reduced to tiny bits. Alternatively, blend chilies with water in a blender or with an immersion blender until thoroughly blended. Add lime juice and season with salt and pepper.
In a mixing bowl, toss shrimp with chili-lime marinade, onion, and cucumber. Season once more with salt and pepper and serve right away with tostadas and avocado.

Look for shrimp that are fresh and have never been frozen, and tell the fishmonger you are planning to eat them raw. If you can't find shrimp that are fresh enough, you can substitute gently cooked shrimp (in which case omit the salt-curing step), or another type of seafood such as raw sea scallops or sashimi-grade fish.
If you buy head-on shrimp, use the heads for another purpose, such as making seafood broth, or deep-fried as a snack.