Travelling with bus from Zurich to Krka is possible via Sibenik, below you can see timetable for buses from Zurich to Sibenik, once arrived in Sibenik you have to take Local bus from Sibenik to National park Krka Waterfalls (Skradin), alternative to travelling via Sibenik is to travel to Zadar, from Zadar there are 3-4 daily buses do Krka (Skradin), timetable you can see here. End to end travel time by bus from Zurich to National park Krka is about 18 hours.