The ocean is at its warmest in September and October because the sea around the island is calm during the summer and is heated by the strong sunshine. Average surface temperatures in these two months is just under 24ºC although shallower areas around the resort beaches are often even warmer.
The sea starts to cool down in November but is still about 21ºC for Christmas.
The sea in Gran Canaria is at its coolest in January, February, March and April but is still over 20ºC in the resorts. The absolute minimum water temperature we've heard about in Gran Canaria is 18ºC and this was recorded in deep water during a scuba dive.
Water at 20ºC is warm enough for swimming when the sun is out although some people retreat to their heated hotel pools during the winter.
The relatively cool sea temperatures around the island are one of the main reasons why the Gran Canaria climate is so good: The cool ocean keeps temperatures here lower than they would otherwise be. Without the sea's cooling effect, the Canary Islands would have the same climate as North Arfica.