Map from Google - Varadero

In Varadero, a town in the north coast of Cuba, equipped with large beaches, the climate is tropical, with a pleasantly warm and relatively dry winter, and a hot, muggy and rainy summer.
Here are the average temperatures.
Average temperatures - Varadero
VaraderoJanFebMarAprMayJunJulAugSepOctNovDec
VaraderoJFMAMJJASOND
Min (°C)191920212223242423232220
Max (°C)252527272930313130282725

Precipitation amounts to about 1,300 millimetres per year; the wettest period, in which it rains more than 150 mm per month, because of heavy showers and thunderstorms, runs from May to October, which is also the hottest period. Here is the average precipitation.
Average precipitation - Varadero
VaraderoJanFebMarAprMayJunJulAugSepOctNovDecYear
VaraderoJFMAMJJASONDY
Prec. (mm)4035507016521015516518017065351340
Days76659131315151287116

Because of its location, Varadero is a quite windy place, which is positive in the hottest months, but should be taken into account with regard to the cool season. In fact, during winter, from mid-November to mid-March, here as elsewhere in the north of Cuba, a cool wind from the United States, the Norte, can blow, associated with cloudiness and some rain. In these cases, at night the temperature can drop to 10/12 °C.

From June to November, Varadero as the rest of the Caribbean can be affected by hurricanes, and in particular the period in which they are most likely runs from August to October.

The amount of sunshine in Varadero is good from March to October, therefore even in the wettest period which as mentioned runs from May to October, because during the long summer, showers and thunderstorms do not reduce the sunshine hours by much, since they are generally of short duration. Instead, from November to February the sun often shines, but there may also be the passage of cloud banks, linked to the outbreaks of cool air from the north, which we have already spoken of.

Varadero

For swimming, the sea is quite warm even in winter, when it's about 24/25 °C, while it gets very warm in summer, reaching 30 °C in August.
Here are the average sea temperatures.
Sea temperature - Varadero
VaraderoJanFebMarAprMayJunJulAugSepOctNovDec
VaraderoJFMAMJJASOND
Sea (°C)252424252728293029282727

When to go

The best time to go to Varadero runs from December to May, although it might be useful to make some distinctions: from December to February, and sometimes in the first half of March, it can get cool for beach life, and there may be some cloudiness and rain: in these cases you may want to do some hiking in the interior of Cuba, to visit the nearby Matanzas and Cárdenas, and perhaps to visit the capital Havana; however, it's better to bring a sweatshirt and long pants. On the contrary, in May the sultry heat begins as well as thunderstorms, but you can still go because the hurricane season has not yet started. All in all, therefore, the best time goes from mid-March to late April.
In summer, for example in August, there's a humid heat, though sea breezes blow; showers and thunderstorms can be intense, but generally do not last long, and there's no shortage of sunshine; however, there is the danger of hurricanes.