May is the best month to visit Marathon. The temperature during the day will be nice 30.0 °C (86.0 °F). The seawater temperature reaches ideal 26.8 °C (80.2 °F). At night, temperatures hover around a pleasant 22.6 °C (72.7 °F). There are only 9 days of rain in the whole month, so you can leave your umbrella at home.