Begin by preparing the agua de ajo by mashing the garlic in a bowl with a fork or masher, add in the water. Stir to combine and set aside.
Cook shrimp with its shell in salted water until fully cooked, about 5-7 minutes. Remove shrimp from water and place in a bowl to cool in the refrigerator.
Once cooled discard shell and devein the shrimp. Place whole in a bowl.
Add in the red onions, ketchup, mayonnaise, lime juice, hot sauce, 1-2 tablespoons of agua de ajo, cilantro, salt and white pepper.
Stir to combine and taste to season. Serve mixture in martini glasses along with saltine crackers.