For raw shrimp: If using raw shrimp, then poach it. Make the poaching liquid by combining water with sugar and salt. Bring to a boil, squeeze a lemon, and then toss the lemons in the saucepan/pot.
Remove from heat, and add the raw shrimp to the hot poaching liquid. Poach for 1-1.5 minutes or until the shrimp is opaque and no longer transparent/gray.
Immediately shock with ice. Allow the shrimp to sit in cold water for 15 minutes, then remove onto paper towels to dry it and peel it. See how to poach shrimp blog post.
For cooked shrimp: Chop the shrimp into small pieces (about ½ inch). In a large bowl combine the shrimp with tomatoes, cucumber, avocado, red onion, jalapeño, lime juice, and lemon juice. Give it a toss, and season with salt and pepper.
Cover with plastic wrap, and chill in the fridge for at least 30 minutes before serving. Serve with tortilla chips or over tostadas.
I used jumbo shrimp (21-25 count) as I like the meaty texture when it's chopped up, but feel free to use smaller shrimp.
Make sure not to overcook the shrimp.
Only add the avocado up to 2 hours before serving.