Hi, as someone who has used scuba to find shells on the southeast Florida coast for 20 years, I can say that many, many species like in the sand 150 to 350 yards off shore, most of these shells which are olives, bonnets etc die and then the shells wash in. So I don't agree that shells found on the beaches are primarily from dredge. a trip to Hobe Sound beach in the fall through winter will produce many shells that wash up there with the heavier wave action of winter. several decades ago I used to find live queen helmets living  within 30 yards of the beach and that was off the village of Gulfstream in Palm Beach County. These helmets live under the sand close to shore or at least they did. early morning, or last light of the day would find the cassis digging out or digging in to the sand. They were beautiful perfect specimens.
I've found many large triton trumpets off shore on the reefs. they usually hang upside down in holes or small caves in the reef, usually where you also find the lobsters or nurse sharks.
all the best

L