The big difference between the North Carolina beaches (I wrote a Frommer's guidebook to the beaches of the Carolinas and Georgia) and Belize is ...
a) water clarity -- the water in Belize generally is, as in much of the Caribbean, gin-clear whereas the Atlantic in the Carolinas is often nearly opague due to all the sand and sediment being kicked up
b) lack of wave action -- Belize is NOTHING like the Outer Banks or even southern NC beaches where wave action is high. There are almost no waves most of the time along the coast of Belize, due to the barrier reef.
c) Belize coastal waters have a lot of seagrass -- again, due to the barrier reef -- and around Ambergris are quite shallow.
Swimming in the sea in Belize is usually not bad from the shore, though there is a lot of sea grass, and once away from the seagrass is fantastic. It can be quite windy and choppy, however.
Belize First magazine http://www.turq.com/belizefirst/