Address: Campus Martius is located at 601 2nd Street. The Ohio River Museum is located at 601 Front Street. The two museums are within easy walking distance from each other. Parking is free at both museums. I would recommend parking at one museum and then walking to the other.
How much time: I would plan to spend a half-day exploring the two museums together.
My favorites: At Campus Martius, I especially enjoyed the Ohio Land Company Office and the Rufus Putnam House, as well as the Appalachian migration exhibit in the basement. At the Ohio River Museum, I loved getting to climb around the W.P. Snyder Jr. towboat. It was fun to imagine being a crewmember and the view was quite lovely.
Truda's Tips: Plan to make a day of it! The city of Marietta is worth a visit too. I had a great lunch at the Marietta Brewing Co.–they brew their own root beer!–and grabbed a Marietta Mocha at Jeremiah's Coffee House, both located on Front Street. Continue down the street to the Ohio Riverfront Park to see the confluence of the Muskingum and Ohio Rivers and watch the barges float by.