While this simple broth often suffices, adding aromatics, such as herbs and vegetables, gives the broth a fuller flavor. Start by sweating the vegetables in butter or oil. A shellfish broth gains a fullness of flavor with an initial cooking of aromatic vegetables, a mirepoix, over low heat. Although you could skip this step and simply combine the aromatics with the shells and the water to make a decent stock, this method will more fully release the vegetables' flavor into the liquid. I usually use one medium carrot and one medium onion per pound of shells.
Leeks are another aromatic option, as are shallots.
Garlic, fennel, and celery are more potent in flavor, but depending on what you're using the stock for, they can be delicious additions as well. Crush the shells to draw out the most flavor. To get the most flavor, especially with the harder shells like lobster and crab, it's important to crush them, breaking them into pieces. A food processor works well for most crustacean shells and heads. It quickly chops the shells into tiny pieces, extracting the most flavor.