I will be sailing through Passau, Regensburg and Nuremberg on a river cruise. Is it possible to do a day trip to Neuschwanstein Castle from either of these ports?
No. It's at least 7 hours round trip by train
Where are you going after Nürnberg? Are you going to Munich to fly from Germany. Spend a few nights in Munich. You can do Neuschwanstein as a day trip from Munich - about 4-5 hours RT.
No, Bavaria's quite a big place and you're too far north to be anywhere near Neuschwantstein.
It could be done in a car with 3 to 4 hours each way - with breaks and traffic around Munich which can be horrible - and queues when you get to the non-castle (it is a modern fantasy, not even a palace) because you won't be able to plan your arrival time.
It would cost a fortune to do that in a taxi, and you don't have the time to rent a car and return it.
It would be a shame to miss Regensburg. It's one of the biggest and best preserved towns of Germany, and one of the few which is really medieval.