Tokashiki Island is good year-round, but the reason to visit changes by season.
January to March is whale-watching season, the single best reason to visit in winter. Water temperatures drop to around 18–20°C (wetsuit recommended for snorkelling), but whale sightings are reliable and the island is quiet.
April to June offers excellent diving and snorkelling conditions — water warming, visibility high, crowds absent. Cherry blossoms bloom early in the Ryukyus (late January to February), before the rest of Japan.
July to September is peak season. The beaches are at their best, water is warm (28–29°C), and conditions are ideal for everything aquatic. The island fills up with Japanese holiday-makers; book accommodation well ahead.
October to December is quieter, with comfortable temperatures (22–25°C) and clear water. Typhoon risk persists through October; check forecasts if visiting early in the period.