The Gulf of Mannar Marine National Park covers around 560 square kilometres of shallow sea between Rameshwaram and the Sri Lankan coast, encompassing 21 islands and the waters between them. The park contains coral reefs, seagrass beds, mangroves, and some of the more diverse marine ecosystems on the Indian side of the subcontinent. Dugongs — large, slow-moving sea mammals related to manatees — are occasionally sighted in the seagrass zones, though populations are small and sightings are not guaranteed.
Glass-bottom boat trips operate from the Rameshwaram waterfront and give a reasonable view of the reef systems in calm conditions. The best visibility is between January and March, when the sea is calmer and the water clearer. Snorkelling is possible at some locations, though the boats and equipment are basic. The park's terrestrial islands are largely off-limits to casual visitors, but the boat trips provide access to the reef zones that make up most of the park's ecological interest.