I'd advise against catching the boat in Mainz ("K-D", by the way.) It lengthens the ride by 1.5 unnecessary hours through some flat and not very scenic territory. The docks are a LONG walk from Mainz's train station. Just take the train to Rüdesheim and begin your cruise where the real scenery begins.
I can only 2nd Russ. Take the train to Rüdesheim and board there. From Mainz there are anyway only a few boats and this only during high season.
From: Frankfurt Airport
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