There are many places where whales, dolphins, whale sharks, stingrays etc. can be seen by a guided boat tour, a snorkel trip or even from the costal line.
The whale watching season starts in November until the end of April. The very best months are March and April for blue whale sightings. The sea is calm in the morning and rough in the afternoon. The whale watching tours start very early in the morning between 6 and latest 7. May to October whales can be spotted, but less in number and the sea can be rough. Anyway, booking is possible at low season, give them a call view days in advance. Be aware May, and June is also the worst time of the monsoon.
At the peak season, the chance for sightings is 90 %. We went there in January and spotted one blue whale a couple of times. We got told that they sometimes spot up to eight in one day. Another good option is Trincomalee from June to October with best sightings in August and September.