During the week the last train connecting with the ferry in Rathen (see FAQ), will be leaving Dresden at 23:00 arriving in Rathen at 23:35.
At the weekend the best connection to Rathen is the 22:00 from Dresden. If you take the 23:00 you will have to change in Pirna and take a bus from there.
Please, check timetables just to be sure. For trains check real-time information on BAHN and for the ferry the site of Kurort Rathen or the timetable on the ferry.