A person's safety from COVID is based on the strength of their immune system, in addition to any breathing or lung issues they might otherwise have. The factors that mediate immune system strength involve age, weight, underlying medical conditions, etc., and therefore are extremely variable by individual. When it is safe for one person to go out is not when it's safe for another to go out. Which means that Disney cannot decide for you when it's safe to sail. Only you can do that, taking into consideration your doctor's advice. It's your responsibility, not theirs.