It's halfway between Baldur's Gate and Elturel, just north of the river Chionthar where it bows northwards slightly.
Edit:
When we crash you get the impression we are on the coast of the ocean or a large lake.
I made the same assumption and spent ages looking at the coastline trying to find it! But if you zoom in so the camera is horizontal when standing at the bits where it looks like a lake, e.g. the cliffs or the edge of the swamp, you can see the land on the other side of the river. So it's big, though not unnaturally so for that stretch of a major waterway.
