Sri Lanka remains a 365 days of the year destination due to its diverse topography.
While the climate is most enjoyable (humid + more cooler) in the drier seasons, the west, south and upland areas are best travelled in the months of December to March. There have been sightings of many schools of dolphins and sharks, so the South is definitely worth a visit.
The East Coast is best experienced between April to September when the waters are beautiful and warm, and iridescent with the colours of coral and pretty fish.
If you are keen to see the best sights and sounds then you have to visit during December and March, when parties are in full swing and spirits are high all over the island.
July and August represent a host of festivals and Perehara Pagents take over the timeline. If you wish to travel during this season, it is recommended to book well in advance to obtain suitable accommodation.
In short, when visiting Sri Lanka if experiencing monsoons in the South, head towards the North/East and you will enjoy pristine beaches and sunny weather, and vice versa.
However, it is a tropical paradise, and there is always the unpredictability of weather in the tropics.
