At the moment we get a debuff for using jump to a charterstone. I think it might be reasonable to debuff extensive use of horse/boat (for some reason I don't want it applied to knarr/wagon or at least in lesser proportions) too by introducing a new wound - sore arse. Longer you are on horseback (or boat bank) higher the chance to get it. While having this wound you can't sit.

As a bonus we'll get a nice icon for this wound.