Tropical sun exposure - not surprisingly - makes a huge difference in unheated pool water temperature. Our pool at Morningside Villa on St. John was exposed to direct sunlight all day long, and the temperature never fell below 85 degrees. Pools with significant shade will cool quickly at night, and don't have time to recover during the day.

By sad contrast, our pool here in Michigan gets direct sun for approximately 3/4 of the day, and it still costs me almost $400/mo to keep it at 85!

Tropics is better.