They spawn mostly during day and yes they are at the lake. Look at the green stuff that it's in the water.
They are kinda rare, I would suggest waking up, going directly to the lake, checking if its there, pass time, checking, pass time, checking and then go to sleep if you have no luck.
It should appear, tho. They are rare, but not like ultra rare. In two days at least ONE of them should have spawned.