Begin by placing 6 cups of water into a saucepan. Add crab boil, rum or tequila, lemon slices, onion chunks, garlic cloves, and coarse salt. Cover and bring to a boil over medium high heat.
Once the mixture has reached a boil, reduce the heat to medium and add the shrimp. Poach for two minutes (and no more). Pour the shrimp into a strainer and immediately cover with ice.
Place the poached shrimp into a glass bowl and add the remaining ingredients. Stir well and cover with an airtight lid or plastic wrap when finished. Refrigerate overnight. While the mixture marinates, occasionally turn the container to move the shrimp in the marinade.