i plan to get there by boat
http://www.battellidiroma.it/en/home.asp
leaves at 9.15 (takes 2.5 hours) arrive 11.30 - Eur 13.00
The last train for Rome leaves from Ostia Antica a little past 22.30.
After that you will have to take a night bus, which stops on the main road outside the station (Via Ostiense),
not in the little road leading up to the station. The stop at Ostia Antica is to the right of this little road.
Hope this helps....