This is a common question that I get asked, and the opinion about it is actually quite confusing in various online forums. Here is what I have found, and what I advise
The latest vision standards are as per DG Shipping MS Notice 1 of 2015.
The actual notice can be found here, and you would need to scroll down to page 16 & 17 for details of vision testing.
The important thing to understand is that there are different vision requirements for joining the Merchant Navy, which are quite strict, and for continuing as a sea farer, which are a bit more relaxed.
Also, the requirements are much more strict for joining a Nautical Engineering course (for Deck duty), versus joining for Marine Engineering or Engine side courses.
Of course, you need to have a normal color vision. Also, the requirements are stricter for Deck hands/ dual career, while for Engineering / Radio the norms are more relaxed.
If you decide to get a LASIK surgery done, you are generally good to go after 6 months or so, assuming there are no complications.
TravelSafe Clinics provides a proper eye examination and DG Shipping certification for both Pre sea examination and for Merchant Navy Seamen, and is approved by DG Shipping at its locations in Chandigarh (Mohali), and Mumbai (Chembur).