rhine river

Most people just take the train back to their starting point, as the current is so strong, you would only have a short time on the ship and then it would have to return, since it takes twice as long going South as it does going North.

To expand slightly on what Jo said, many of the towns on the Rhine (especially middle Rhine) hug the river. They have a dock where you board or debark from your boat. Two blocks or so away is the town's train station, where a local train (known as Regional Bahn or RB) runs about once an hour. You can set your schedule based on which way you chose to cruise the river. For example traveling between Bacharach and St. Goar (a popular stretch) is roughly 14 km by train (so no more than 15 minutes). By boat, it depends on whether you go with or against the current, and time of year (how strong the current is). As a general rule, I say the boat trip is "about an hour". In truth, it's more like 45 minutes downstream and 1 hour 15 minutes upstream (again, depends on the current). That's a long way of saying you can plan your trip to use most of your available time on the boat, then catch a train back to your start. If you time it right, you won't wait long for the train. If you have to wait, use the time to look around town before getting to the station.