In tropical regions, such as the Caribbean, the average seawater temperature is around 79-82 °F (26-28 °C), while in more temperate regions, such as the Mediterranean, the average water temperature is around 64-72 °F (18-22 °C).
Geographically, the Bahamas is located in the Atlantic Ocean, while the majority of other Caribbean territories are located in the Caribbean Sea. Although both are considered warm-weather destinations during the summer months, the Bahamas becomes more temperate in the winter.
The temperature variations in the Caribbean are surprisingly slight, averaging between 75°F and 85°F (24°C-29°C) in both winter and summer. It can get really chilly, however, especially in the early morning and at night. The Caribbean winter is usually like a perpetual May.
The warmest region throughout the year is the south-east Aegean: Kos, Rhodes, Bodrum and Marmaris. Except for June and July where also the Ionian, Saronic and Cyclades areas are as warm as the south-east Aegean.
Water is exchanged at the slowest of rates making the med as clear as a swarovski crystal. Limited water exchange = limited food for tiny organisms called phytoplankton aka algae, and it's these little dudes who play the lead role in water clarity. The Mediterranean is classified as “oligotrophic”.
The best time of year to visit is from December to May. Temperatures at this time are between 21 to 24 25°C (70-75 F), and the risk of tropical storms is low. Swimming year round is also possible however, because seawater temperatures usually stay above 25°C (80 F).