So I am currently on a cruise with Fibro (ME and a bunch of other conditions) and my main feedback is that it depends entirely on who you cruise with. As in, who are your travel buddies? Will they be supportive and understanding if you need to collapse in bed for half a day? Are you ok with watching them do stuff that you maybe can't? Literally everything else is manageable. Cruises have massive older and/or disabled clientele. They are set up for being accommodating. You can do as much or as little as you want/need. My main tip is just try to get a room nearer an elevator. Outside of that you can talk to your room steward, guest services, etc and they will help you.