Dumb question, but if we visit this area, can we easily walk to a beach or sea area? I read in the guidebook that part of the ruins is the port that used to be at the sea but is now 1 mile away--are there beaches or water viewing area nearby?
It's not a dumb question at all. You wouldn't want to walk to the beach from Ostia Antica (it's 2 1/2 miles and not pleasant), but you can get back on the train that dropped you off and continue into Ostia Lido. There are a couple of stops that have a "wavy" symbol that means they're within walking distance of the beach.
Probably the best stop would be Stella Polare. Walk straight out of the station and keep walking southwest until you hit Piazza Sirio and you'll see the beach.
Or the Castel Fusano stop would be another option.
Ostia Lido, the beach area, is a fair walk but only 2 short train stops (about 5 minutes) beyond the train stop for Ostia Antica. A 24 hour transit pass (4 euro) will cover the entire trip from central Rome, and costs the same as the 4 individual tickets you'd otherwise need.