What should I expect in Sri Lanka in late September and early October: rain, sea conditions and available activities?

The ocean was described as less calm during the shoulder season.

The discussion reported that not all cafés and restaurants are open during the shoulder season.

According to the response, not all excursions are available during the shoulder season.

In response to a question about visiting from September 18 to October 2, the discussion reported that shoulder-season rain in Sri Lanka generally does not disrupt a holiday.