It's called the Palk Strait.

The Palk Strait and the Gulf of Mannar separate the two countries.

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The Palk Strait is what separates India from Sri Lanka. A Strait is water that conects 2 oceans together, in this case the Bay of Bengal and the Indian Ocean are connected by the Palk Strait, which also separates India from Sri Lanka.

Andaman Sea

Palk Strait is a strait between the Tamil Nadu state of India and the northernmost part of the island nation of Sri Lanka. It connects the Bay of Bengal in the northeast with the Palk Bay and also with the Gulf of Mannar in the southwest.

The Palk Strait.

The bow

palk strait