Now coming to the answer of your question the minimum eyesight required to join a merchant navy college are as follows:-
Deck Side:- 6/6 in both eyes.
Engine side:- 6/12 in both eyes or 6/9 in better eye and 6/18 in the other eye.
The above mentioned requirements are the same for getting CDC.
Now for boarding the ship of your company the minimum eyesight requirements are different. They are as follows
6/60:-unaided for deck and engine side
6/15:- aided for engine side in both eyes
6/12:- aided for deck side in both eyes
Source of above mentioned information is table A/9 of STCW.