1. 1. Peel the grapefruits over a strainer and let them drain. Shell the shrimp, reserving 6 whole ones.
2. 2. In a bowl, mix the cream, ketchup, and vinegar. Season with salt. Add the grapefruit flesh and shelled shrimp.
3. 3. Choose 6 glasses, line the bottom with a bed of lettuce, then add the mixture. Garnish by placing a whole shrimp in the center. Chill for an hour before serving.