Remove shrimp heads and shells, reserving both. Set peeled shrimp aside in refrigerator for use in another recipe. Hold 1 steamed crab with both hands, and insert a thumb under the shell by the apron hinge; remove top shell, and set aside. Repeat procedure with remaining crabs. Set aside crabmeat in refrigerator for use in another recipe.
Combine shrimp heads and shells, crab top shells, water, carrot, onion, celery, and salt in a large stock pot. Bring to a boil. Reduce heat; cover and simmer 2 hours. Remove from heat, and cool slightly. Strain stock through several layers of damp cheesecloth, discarding solids. Use Seafood Stock as a foundation for gumbos and stews.