Zanzibar is famous for it's incredibly huge shallow and sandy lagoons between the beach and the reef.
There are two windy seasons: One from December to March, the other from June to October. Best months are usually January, July and August. In these months you can expect around 80% windy days. In December, February, March, June, September and October you may expect 60 - 80%.
Typical wind strength is around 18-20 knots in the peak month (in the other months it may be a bit less). The winds tend to be quite consistent with it gradually building, then decreasing in strength over a week or so period.