Acknowledging that the identification of a qualified motor vehicle is strikingly more detailed than the identification "boat," and acknowledging that the provision as written would seem to apply to a craft that sails across the water powered by wind but apparently not a glider that sails through the air powered by wind, and acknowledging that "motorized" would be a conceptually attractive distinguishing characteristic, I would, as is my wont, approach this analysis through the lens of a different question, namely, "Why is this provision in the tax code/what was Congress trying to accomplish?"
My understanding that Congress was trying to address a perceived disconnect in the tax code, whereby a donor would realize a large tax donation when donating a personal conveyance of some ilk, based on a possibly-inflated fair market value, while the charitable recipient would in turn actually realize a categorically smaller benefit when they sold such contributed property.
From that perspective, it seems like gifts of sailboats would have been squarely in Congress's cross-hairs. Even in decidedly non-nautical educational institutions I ran across donations of sailboats that had exactly that characteristic of large-deduction, small-benefit (often with some related-use considerations thrown in as well).
So while it might have been better if the law had been drafted with more detail, and may as a result sweep up your $600 kayak (not sure whether Congressional members or staff could have conceived of a $600 kayak in 2004), I'm pretty sure that some non-motorized boats would have been intended as part of the targeted vehicle class.
My US$0.02 worth; the usual disclaimers apply.
Alan
Sent from my mobile device; please excuse the inevitable typing/autocorrect infelicities.