I'm usually in Belize in August and last August was at both Ian Anderson's and Black Rock (actually I stayed at Ek'Tun which is across the river from Black Rock -- Ek 'Tun by the way is fabulous). I've always had pretty good luck with August weather -- it's hot and humid but that's how Belize is most of the time. I don't think I saw a mosquito at Ek'Tun -- there aren't many in that area due to the porous terrain which doesn't allow much standing water for mosquitoes to breed -- and only a few at Cave's Branch.
On antimalarials we've had many discussions on that issue here -- just search the board using a key word such as malaria.
Belize books and publications by Lan Sluder:
Belize First Magazine (http://www.belizefirst.com
Fodor's Belize & Guatemala Guide (new 4th edition coming this summer)
Adapter Kit: Belize (the first and only comprehensive guide to living, retiring, working and investing in Belize -- a best-seller on Amazon.com)
San Pedro Cool (the first and only visitor guide to Ambergris Caye)
Belize First Guide to Mainland Belize
UpClose Central America
Belize Book of Lists