I was diagnose with mg and was put not fit for duty as a merchant mariner, what does that mean and should i look into another career?

Yes to new career: 'not fit for duty' is pretty straightforward phrase--identifies presence of a medical condition that is of adequate severity as to pose risk to your health/life and as such is incompatible with functioning as a merchant mariner. Determination is usually permanent, as most medical conditions do not go away. You will need to find a new career field with more reliable medical access.

Myasthenia gravis: Mg is a neuromuscular disease that causes muscle weakness and fatigue. Medication helps but it is still difficult to have a career that involves heat, physical labor or lacks ready access to medical care. The severity varies from person to person so it is best to ask your doctor what you can expect.