We just made the journey to Santa Marinella during our stay in rome and thought the info might be useful to someone else. The beach was beautiful here and we were very glad that we made the daytrip out to enjoy the water!
We could not use the automated ticket machines at Rome Termini Station for some reason but had to go to the counter and buy the tickets: 4.50 Euro each, each way and children (under 4 I think) are free.
Note to get to Termini early as the train leaves from platform 28, meaning we had to RUN out 1/4 mile or more to the end of the station to make our train. Trains leave every hour or so. You have to stamp your ticket in a green machine before you board the train. It is about an hour ride, but very worth it.
We arrived and walked out of the station, walk a block straight, then turn right and walk a block...you will see the beach on the left side. The free beach is on the sides, you have to pay to be in the center (someone will come up and offer you an umbrella or chairs (about 10 Euro each). We were glad we did once we saw how crowded the free beach gets at noon. Bring your towels, did not see any lockers, but we didn't ask either. There is a small snack station above the beach.
When we left, we stopped at the bakery close to the train station (excellent canoli!)
If you have any questions, I will try to answer.