The best time to travel to Cuba is between November and April. This is the dry season and the average temperature is around 25°C. In January there can be some cooler days. This graph shows the average monthly temperatures of air and water:

The website of the Meteorological Institute always shows current information on the subject of weather (only available in Spanish):
www.met.inf.cu/asp/genesis.asp?TB0=PLANTILLAS&TB1=INICIAL