No-see-ums or sand flies or sand fleas are fairly common in Belize. One of the worst areas is probably around Hopkins/Sittee Point, but they can be a nuisance on Caye Caulker, on quite a few of the remote cayes and on the Placencia peninsula and just south of there on the coast. But they can be bad almost anywhere there is sand or grassy areas near the water, when the wind isn't blowing.
I have never found them to be bad on Ambergris Caye, however.
They are bad in the Bay Islands of Honduras. But some of the worst I've ever seen are in coastal South Carolina and Georgia.
DEET helps some people. Others swear by Avon Skin So Soft or baby oil or other oil-based lotions which tends to drown them before they can bite you.
Belize First Magazine http://www.turq.com/belizefirst/