How to get from Amsterdam airport to Rotterdam cruise terminal using public transport
It's very easy, there is a train station within the airport and direct trains to Rotterdam central. From there, you take metro D or E towards the south (that's any destination except “Den Haag Centraal”) and get off at Wilhelminaplein. The cruise terminal is close by.
During the day there are several trains an hour (6 or more, 4 of them “Intercity direct” trains, which are faster but require a €2,40 extra fee) and a metro every few minutes. Trains run through the night (less frequently) but the metro stops around 1 or so.
For exact timetables or alternative routes, you can check 9292.