Sri Lanka is a tropical island, just above the equator. The weather is hot and humid all year round. Sri Lanka is also a conservative and deeply religious country so the correct attire, dress guidelines and etiquette must be observed.
Light cotton clothing is always recommended. Bikini tops and ultra short pants are not always a good idea, especially for women.
In and around Colombo, you can wear what you wish and be as daring as you like. But in the outskirts and smaller towns it's better to be conservative and wear longer knee-length pants and t-shirts.
When visiting religious attractions, women and men must be covered past the knees. Some places of worship such as the Dalada Maligawa supply sarongs for visitors, but not all attractions offer such services.
There are no nude beaches in Sri Lanka. There are also no private beaches in Sri Lanka. Every beach is public. When within the confines of the resort you may wear that you like and sunbathe topless. But at the beach you should not go topless unless you want to attract unwarranted attention.
