Yes - be careful. Every cruise line I have ever worked for gave me the examination after I arrived. However, I admit that was a long time ago.
It sounds like you are going through employment agencies who may be asking your for money to help.
You may be getting scammed. Please ask for the credentials of anyone who says they can get you a job - and double check everyone you meet with a google search.
We have seen people getting scammed - and we try to document it whenever we can.
By the way, people, you can go to a good doctor who can give you a complete physical. If you are indeed healthy and disease free you can just make that report a part of your resume. It won't hurt your chances.