I have not taken the river cruise, but I have taken the train, which runs along the river. I highly reccomend this trip.
It is probably a 5-10 minute cab ride to the station in FRA Mainz. It seems like the train ride is 90 minutes or so. And it drops you off just near the Dom in CGN. Another 5 minute cab ride to the InterContinental.
I suggest you go to the grocery and get a baguette, 1/2 pound of cheese and a bottle of wine to help you deal with the stress of Deviating!