In December in San Blas, there are ideal conditions for a vacation. The temperature during the day will be nice 30.1 °C (86.2 °F). The seawater temperature reaches ideal 27.6 °C (81.6 °F). At night, temperatures hover around a pleasant 23.7 °C (74.7 °F). There are only 9 days of rain in the whole month, so you can leave your umbrella at home.