The waters off Mirissa on Sri Lanka’s southern coast lie above a deep ocean trench that blue whales and sperm whales follow on their migration routes through the Indian Ocean. This full-day experience departs at dawn on a purpose-built whale watching vessel with marine naturalist guides, heading into the deep water where blue whale sightings are genuinely reliable from November to April. The spinner dolphin pods that accompany most departures provide company on the way out and back.