Orthostatic Intolerance? POTS? If you are seeing a cardiologist or neurologist for care and have a diagnosis, then you are well within your rights to request special accommodations. I would be providing a letter from your doctor to the cruise liner prior to departure so that you discuss any appropriate accommodations.
I have POTS, and personally, I don’t feel the need to “push in”, but if a chair could be placed in the shade, at check in, I would happily wait until it would have been my turn should I have been able to stand in the queue. I do this whenever I have to queue. People are understanding once the realise that you don’t want jump the queue, your body’s thermostat just doesn’t work properly!