I agree it would be good if the NPCs used the same rules. However, I don't think it's that easy to get it to work properly:
- Villagers would have to get skis
- NPCs would need to know when and how to use skis and wield/unwield the ski stick (they know how to wield/unwield shields to fire bows, though, so there may be an existing base to build on).
- Once animals know how to use their innate ability to swim it might be time to get humans to learn when and how to swim. However, humans have to learn to swim, and most probably didn't know much about swimming.
- Swimming out to a craft to attack the user seems like an incredibly stupid move to me. Not only is the craft probably faster, but the person on the craft should have a big fighting advantage from being higher and having a footing (rather than literally having to push off of water).
- If NPCs are taught how to move as the PC does, they may also steal your water craft and travel on water craft themselves.