Sri Lanka Tourism describes November–April as the calmer beach season; May–October can bring rougher seas. Whale and dolphin trips leave Mirissa Fisheries Harbour around 06:30 and usually last 3–4 hours, depending on sea conditions. Sightings are never guaranteed.
One provider, Mirissa Whale Snorkel, lists its Whale Watching Tour at USD 45 per person; children under 5 travel free. This is that operator’s price, not a general local tariff. Verify inclusions, departure, cancellation terms and conditions before booking: https://mirissawhalesnorkel.com/pricing
For responsible wildlife watching, check the boat’s safety equipment and briefing. SLTDA guidelines set a 100 m no-approach zone and discourage swimming or diving with whales or dolphins. Follow crew instructions; do not touch or feed marine wildlife: https://www.sltda.gov.lk/storage/common_media/GuidelineForWhaleandDolphinWatching.pdf
Photo: Tharakaliyanage, “Mirissa Beach Sri Lanka”, resized and converted to WebP; CC BY-SA 4.0: https://creativecommons.org/licenses/by-sa/4.0/; source: https://commons.wikimedia.org/wiki/File:Mirissa_Beach_Sri_Lanka.jpg