Two separate issues, Airbnb is in the hosting business and perhaps is best to keep that aspect separate and as simple as possible. Kayaks or such supportive items are like a rented car, are not part of staying in the listing. We rent all kinds of things, and our guests go on all kinds of tours, we collect for such things outside of Airbnb to not complicate matters.
I could just imagine calling Airbnb and trying to collect from them for damage to a kayak (a bit hard to do) that is being used in a lake that has little to do with the actual property.