First prepare the water bath-- put about 4 quarts of cold water into a big bowl and add ice and sea salt. Stir the mixture until the water is ice cold and some of the salt has disolved.
Add another 4-5 quarts of water to a large pot and bring to a rolling boil.
Add all the shrimp at once and give a slight stir.
Adding the shrimp will stop the boil for about a minute or so, when the water starts to boil again and the shrimp change color and start to float to the top, take them out right away-- don't overcook!
Remove as fast as possible with a slotted spoon and add them directly into the cold water bowl. The water should still be cold even when all the hot shrimp are added-- if not, add more ice!
After 15 minutes drain the shrimp and serve, or put in the fridge covered with a moist paper towel for up to one day.
In Spain we serve as is, sometimes with sliced lemon. People peel them with their hands and don't worry about deveining. If you prefer to shell and devein before serving you certainly can, but it's really not necessary!