The deceased comprise 10 boys and three girls, police said.
Altogether 18 students had gone to swim and five were rescued, a local revenue official said.
The students were part of an around 130-strong picnic group from Inamdar College in Pune. They had gone to Murud in three buses.
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Meanwhile, the Coast Guard has launched search by an
interceptor craft and a Chetak helicopter to help bolster rescue operations.
A Sea King helicopter has also been pressed into service, a senior official said.