As he did not know swimming initially, he had to learn it but while shooting, he was overcome with fear. Co-star Varun Dhawan (who is a good swimmer) realised it and swung to his rescue. A doctor was summoned to see if Sahil was fine.
Even though the crew didn't want to shoot and take a chance, on the persistence of Sahil they went ahead with the shot and the crew applauded the efforts of the newbie for overcoming his phobia.