Explain to all the medical personnel that you see that you do not want to leave the Navy. Start compiling information about your detached retina and find out what options are available to you (surgery etc), or if you can recover over time. You can then start to build an argument to keep you in the Navy rather than discharge you. They won't want to discharge someone who is committed to recovery and the rest of their career. You can also ask for second opinions. If you try and fight it, then you may win and they might keep you in.
A friend of mine recently picked up an infection in his eye in Afghan which damaged his sight and he was threatened with possible discharge, but he has an exemplary record and showed his will to carry on with his career. He also sought the opinion of an eye specialist who did tests and confirmed that his eyesight would improve again over time. He's still in the RAF reg now and not even downgraded anymore.
Good luck to you mate, medical discharge is hard to deal with
I've been through it myself.