We'll be in Rome for 9 days. Want to go to Venice for the day and another day go to the ocean. How do we do that by train? We are staying down the street from the Colliseum.
Don't bother to do a day trip to Venice. You will have too little time to enjoy the uniqueness of Venice.
Can't you take a few days away from Rome and spend at least two or three nights in Venice? It's too far for a day trip, but 9 days is quite a long time for just Rome.
The ocean is in Portugal or Spain or France. Too far from Rome for a day trip. Can the Tyrrhenian Sea do? Ostia Lido is just 20 miles away.
Its 3 3/4 hour each way to Venice, so that is 7 1/2 hours on a train. Take the first of the day and come home on the last, leaves you 8 1/2 hours in Venice. An advance purchase same-day-return ticket is 109 EUR.
The ocean, just take a Regionale to Ostia. Cheap, quick and easy.
Roberto is correct, no ocean near Italy. Ostia Antica and Ostia Lido would make a good daylong combination.
You should spend two nights in Venezia if you can fit it in.