Fill pan 3/4 full with water and bring to a rolling boil. Add shrimp to pot and stir twice. Immediately drain the shrimp into a colander or remove with slotted spoon.
Rinse shrimp in colander with cold water to stop the cooking process.
Add lemon and lime juices to a large bowl. Add shrimp to the bowl and allow to sit, submerged, for 10 minutes.
Remove shrimp with slotted spoon to another large bowl. Do not discard citrus juices.
To the shrimp, add remaining ingredients and using gloved hands, toss well. Add about 1/4 of the reserved lemon and lime juice and toss again. You can discard the remaining juices at this point.
Cover and chill in the refrigerator for at minimum 30 minutes, or up to 8 hours.
Notes
Fresh citrus juice is much better than bottled when it comes to this recipe. The acid in the juices is what will actually help cure the shrimp. For 1 cup of lemon juice, you will need about 5-6 lemons. For 1 cup of lime juice, you will need 8-9 limes.
Shrimp cooks very quickly. The process described above will poach the shrimp and then when placed in the citrus juices, the shrimp will continue to cure. Be sure to rinse in cold water as directed.
You can prepare this dish 6-8 hours in advance and store it in the refrigerator until you are ready to enjoy. I would recommend holding off on the avocado until you are ready to serve to avoid it from browning.
Store any leftovers in an air-tight container kept in the refrigerator for up to 2 days.