Bring a small pot of water to boil, add the salt and stir. Cut the lemon in half, squeeze the juice into the water then drop the lemons in. Add the shrimp and bring back to boil. Boil for 2-3 minutes.
Take the shrimp out of the boiling water and drop into an ice bath to stop the cooking process. Drain on paper towels.
Add the shrimp, crab, red onion, celery, mayonnaise, lemon juice and seasonings to a bowl and stir together until combined.