I did the big Rhine River cruise today....from Wiesbaden-Bebrich to Saint Goarshausen. I'll offer ten bits of advice.
1. The cruise line that comes over to Wiesbaden-Bebrich is KD. You can Google them up, and find that they run around 9AM each day. At the Bebrich Port, you need to be there at 8:45 (it leaves at around 9AM)....to buy tickets and get into place. Note, there really isn't any parking at Bebrich, so calculate about an alternate parking scheme or use public transportation to get there. Rough cost of a one-way ticket to Saint Goarshausen is 35 Euro (adult).
2. On a hot day, bring a bottle of water along. This trip will take three hours to get over to Saint Goarshausen. The KD ship has beverages but you will generally pay more than normal.
3. Weather is always a factor, so know the situation for the whole day prior to getting on the ship. While there is an interior.....it's a major rain episode.....it's not worth a trip.
4. Weekends are the worst period to ride the cruise vessel.....with lots of tourists showing up.
5. There are roughly a dozen places where the vessel will stop and you can get off.....so making a three-hour trip isn't always necessary. And the lesser the distance, the cheaper the trip.
6. A big note about Saint Goarshausen, there are only three or four restaurants in the small town, and I'd generally give them a marginal three-star rating, with two decent coffee shops. As the vessel drops you off.....if you point from that exit to the 11:00 position, that's the railway station in a three-minute walk. Cost for an adult back to Wiesbaden is 12 Euro. My recommendation is for you to bring along a sandwich....have a cup of coffee and slice of cake at the coffee shop....and feast out at the end of the day. The train leaves every thirty minutes and takes about one hour to reach Wiesbaden.
7. Don't bring kids under the age of eight along.....they will get nothing out of the trip and likely be bored. I'd even hesitate bringing any kid under fourteen on such a trip unless they show some enthusiasm.
8. The cruise boat rarely rocks, so don't worry about sea-sickness.
9. Bring sun lotion along if it's a cloudless day.
10. It's windy on the river so dress appropriately....365 days out of the year. Note that they typically only run the vessel from late April to late September.