The medical eye standard for DNS course in India is 6/6. If one has eye sight of 6/9 after DNS course, will there be problem ?
While it is true that medical standard of eye sight test for joining Merchant Navy after 12th, needs an unaided distant vision of 6/6 in better eye and 6/9 in other eye for Deck Department, however for periodic medical tests STCW 2010 and MMD allows spectacles ! Criteria later changes to aided vision !
In short you can continue your career wearing spectacles without much issue. Attaching Annex III (minimum in service eyesight standard for Seafarers) below, hope it helps you !