The humidity aspect is surprisingly complicated, but a key thing to keep in mind is the amount of water vapor in air is never that much - maybe a few percent depending on temp. Air is around 78% nitrogen and 19% oxygen with the remaining 3% or so can be a lot of trace gases and water vapor. But regardless of the change in water vapor content between bone dry air and saturated air, it's not going to swing the general heat convective properties of the gross fluid THAT much. The fact that the literature on changes in convective cooling due to humidity is so sparse would lead me to believe that the affect is generally small enough to be ignored.

That's not to say that water vapor doesn't have a huge affect on how the atmosphere behaves, but this is due to the relatively large amounts of energy involved where water phase changes between liquid and gas - that's not a factor in the convective cooling of a water bottle in an air stream which is more/less determined by the gross properties of the stream (e.g.,its density).

Actually, come to think of it, that's another factor which might be playing a bigger role than you might think - was the bottle externally wet before the ride started or did the bottle become wet during the ride? Evaporative cooling is a pretty big deal.

- Mark