A crab boil, while similar, isn’t quite the same thing. So I wouldn’t be looking to model it on something that happens in Maine. It’s also a misnomer, because little liquid is actually used, and the contents are also steamed rather than boiled.
The main difference is that a crab boil is done in a pot, using heavily flavored liquid. Indeed, there are crab boil flavorings available in any grocery, in both liquid and powder form.
Why seawater? Other liquids not only add flavor, they can be used as the base of a sauce. For instance, let’s say you use beer as the liquid and Old Bay as they over-riding flavor base. You can use that, mixed with melted butter, as a great dipping sauce for just about everything that comes out of the pot.
That said, I would definitely add some sausage to your crab boil; any good smoked sausage will work, but, personally, I’d go with chorizo.
A good number of crushed garlic gloves would be an excellent addition. Several bay leaves wouldn’t hurt, either. You want to boil the water/broth at least a few minutes to get the seasonings well-infused before you drop the crabs.
A different approach is to make a slurry of room-temp butter and large amounts of garlic, then boil the crabs until they’re about half cooked. Let them cool enough to handle, then break them up and thoroughly crack all the shells. Then massage the garlic-butter slurry through the cracked shells and finish on your grill. Mesquite smoke is good with this, though I’m sure alder would do fine.
You want the pot at a simmer, not a boil. Add the sausage first so it can contribute to the pot. Give it ten minutes then add the potatoes. Another fifteen, and add the cleaned crab. Five more and the corn (and chunks of fish if you’re using fish). Another five, and add the shrimp and clams and/or mussels. Turn off the heat, let it steep, and in exactly eight minutes and 12 seconds — give or take — the clam and/or mussel shells will open, the shrimp will be cooked to pink perfection, and unless your friends are like mine there should still be cold, refreshing beverages.
Hammers for pounding the crab.