Yes, you are correct Kalpitiya is one of Sri Lanka’s whale watching hotspots. The best time to spot whales in Kalpitiya, is between November and April when the seas are relatively calm. The peak movement of whales is between November and December and again between March and April.
Note: The ideal locations for whale watching would be Dondra Point which is accessible from Galle, Hikkaduwa, Mirissa and Trincomalee while the sea off Kalpitiya teems with an abundance of dolphins.
For more info Whale Watching