Cayman Islands Average Temperature By Month - Grand Cayman Island Vacation - Travel Guide 2021 . Today's north side, cayman islands water temperature. Average monthly temperature, 2018 2014 monthly rainfall totals for gt, 2018 average monthly relative humidity, 2018. The temperature in the cayman islands, summer or winter, seldom goes lower than 70°f or higher than 90°f with the daily average varying between 77°f occasional cold fronts pass through the islands in the winter months, the remnants of more intense winter weather moving south through the. Naturally, this is also the most expensive season, and reservations must be made at least three months in advance. Heat advisories are in place across portions of the northeast states through monday due to the combination of hot temperatures and high humidity.