At times of the year, especially in the late summer after a lot of rain, away from the water and breeze, mosquitos on North Ambergris and also south of the developed area on the south part of the island can be really bad. Really bad. If you're on a golf cart and even slow down a little the mozzies can cover you in seconds.
I once saw a woman coming back from North Ambergris on the ferry across the channel whose back was literally covered with hundreds of mosquitos. She was carrying a little baby and didn't want to use a repellent around the infant. She must have been in real pain that evening, from all the bites.
But in San Pedro and in the developed areas around it mosquitos you see few if any mosquitos.
Few visitors to Ambergris Caye bother with anti-malarials, and I have never heard of a visitor getting malaria on the island, although I believe that the CDC still recommends chloroquine for visits anywhere outside of Belize City.
Belize First Magazine http://www.turq.com/belizefirst/