Hey Sean, I was in Mainz two weeks ago. We went on a Rhine cruise from Mainz to Koblenz(except we never made it to Koblenz). Go to the river by the river Hilton and there is a Kiosk there where you can buy a ticket. The first boat leaves at 9:45 am. You can take it all the way to Cologne, but you will have to change boats in a town that I can't remeber the name of. The cruise was great, but you stop in a lot of towns along the way, soo it will take you a while to get to Cologne. If it's a nice day you will have a great time. Good Luck.

Rich