In the snapshot 16w03a and previous, the hitbox of the horse was larger than the hitbox of the boat (the boat sink more and the horse's legs is out for under the boat), making it impossible to break the boat if a horse is riding passenger (bug report MC-89963 ). In 16w04a, it was solved thereby. If a mob's hitbox is too big it won't go into boats, the horses can not be mounted on the boat (bug report MC-96212 ) but, in the snapshot 16w04a / 16w05a, if you get put a horse in a boat (for example, with commands), now you can interact with the boat. The boat hitbox is below the horse hitbox (you hit the boat down and the horse will disembark because the boat does not sink and the horse's legs are inside the boat). So, it is not necessary that the horses can not be mounted on the boat.