The winter season (December through April) is the peak period: warm, dry weather, calm Gulf waters, and the steady influx of snowbirds from the northern US and Canada. Prices are highest and accommodation books up well in advance. This is the best time for beach-going and wildlife watching, with manatees at their most concentrated in the warmer bay waters.
Summer (June through August) is hot and humid, with afternoon thunderstorms almost daily. Crowds thin significantly and prices drop. The water is warm enough for comfortable swimming and the shelling is excellent after storms. Hurricane season runs from June through November, with September and October the riskiest months — always check forecasts if visiting in the fall.
Spring (March through May, after Easter) offers a sweet spot: crowds dropping from peak, warm weather, and still-pleasant Gulf conditions. Many long-time visitors consider May the best month on the island.