I moved to Tokyo along with my husband & sons shortly after being diagnosed with ITP and on 60mg of prednisone. Once in Tokyo I had a fantastic hematologist who knew my state-side hematologist (small world). Later we moved to Hong Kong and that hematologist didn't know much about ITP and would panic if my count dropped and would want my spleen removed - no way! We traveled to Thailand, Mainland China, Macau, Italy, the UK, Singapore and more. I've also been on various cruises since diagnosis, including a river cruise in the US and Europe, trips to Ireland - and to be honest I didn't know what my count was before o any trip, just the moves. Know I am not answering your question but just thought I'd let you know that ITP isn't the end of everything - you can still live.
Is this a river cruise? Do you have a hematologist, if not you should have one, if you do then talk to him/her about your trip. The 2 of you can come up with a solution for your trip. I would think a cruise ship would have medical onboard and probably could do a blood test.
"Instead of wasting your time worrying about symptoms, just get it checked out" -Nieca Goldberg, MD