Pleuroploca gigantea -
aka Florida horse conch.
The Florida horse conch is the largest snail to be found in the American waters, sometimes reaching a length of two feet. It has ten whorls, and its shoulders bear large, low nodules. The operculum is a leathery brown color, the aperture is orange, and the animal itself is brick red in color. They are carnivores that feed on bivalves and other snails.
Florida horse conchs are used as food and are said to taste "peppery."
Thanks Mike for the positive id and all who wrote in with suggestions.