Mainz Hilton will be able to hook you up with a schedule. Use the IC or EC trains (NOT the ICE - intercity express). It's a couple hour ride for about 80 bucks - first class (50-60 euro if I remember right). You can just board and pay with the company credit card on the train. Easy walk to the bahnhof (train station) from the Hilton. I'd get a taxi on the CGN side just to make it easier. Worth the effort if the wx is nice.
No gouge on boat ops.