Yes - our experience has been that it is a flat fee for the initial deviation, regardless of whether it affects only outbound or both inbound and outbound. (Once upon a time, they would do a deviation fee for just one leg of the trip that was like half the cost of the 2 way deviation, but they don't do that anymore - it's either all or nothing!).

Be advised that if you make an initial deviation and pay the fee, and decide to do something different later on, you will have to pay an additional deviation fee (just like you have to do with airline change fees). This happened to us on our cruise last spring from Mumbai to Rome.