Did you do this ? If so was it a good idea.
Sidneynick's point is a good one. It is doable, as we (and countless others) did it one time off a cruise ship in nearby Civitavecchia, and the rides were about an hour each way. And local commuters do it every day to jobs in Rome. So if you don't want to stay in one of the world's great cities for any of many reasons, you could do it.
I love staying right in Rome, though.
Why would you want to do this?
As pointed out, you'd be spending a more than 2 hrs a day in transit. Santa Marinella is ok as a beach resort, but it's not a very interesting town compared to some other options.
So it depends on what you're trying to get out of this but it's not something I'd personally would ever consider.
Our plans have changed somewhat but we still are flying to Rome & then going straight to S Marinella for 3 nights. We'll go to Rome at least for 1 full day. Then off we go to several other cities in Italy, then back to Rome, staying in the city several days before we head up to Lake Como, then Milan. I do realize there is a train commute but we are attempting to "be Italian" for 3 weeks so thought it might be relaxing for the first few days. We go in October. Thanks to everyone for information and encouragement