You can still bring the scrying stone back to the priest (outside the church in the village at the start of Broken Valley) who gave you the quest to complete it.
The lake shrine is not involved in the quest itself, it is just a fast way to get to that dungeon, or leave the area once you are outside. If you want to explore some more, or still need to go back to talk to Lovis, or whatever, you can wait to turn in this quest.
