I had never heard the ONCE IS ENOUGH before. Things I have read have said a booster shot every 10 years.

If that was what Regent said they will let you board for sure. HOWEVER I am going to check that detail again as often misinformation can happen when you call and get a representative. If we get that cleared then you should be fine. The different criteria may depend on the countries you visit and the likelihood of you contracting yellow fever and I think that the chance on a cruise ship is very small.