With a coarse vegetable brush under cool running water, scrub the fish bones clean, scrub away the blood lines and place the bones in a 12-quart stockpot.
Rinse the crabs under cool running water.
Break them in half and use a kitchen mallet or the edge of a large sturdy mixing spoon to break up or crush the shells as much as possible.
Add the crab shells, the shrimp shells and the shrimp heads to the pot with the fish bones.
Add the onions, celery, carrots, tomatoes, garlic, parsley, peppercorns, bay leaves, sea salt and thyme.
Pour the water into the pot, covering all the ingredients with the water.
Over high heat, bring the liquid to a boil.
Lower the heat to maintain a gentle simmer and cook, uncovered, for 40 minutes, skimming any foam or coagulants from the surface as they develop.
Replenishing the water as the liquid evaporates.
When the simmer is complete, strain the stock through a fine-mesh strainer.
You should end up with about 6 quarts of strained stock.
If you have less, make up the difference with water.
Serving Suggestion: Use the stock immediately or allow it to cool down and store in the refrigerator overnight or in the freezer for up to one month.