One good way to tell a river from a sea is to look at the way the water flows. Seas and oceans have tidal movement, perpendicular to the shore. At least on Earth that is the case. I don't know if the fictional planet on which BG3 is played out has any very large moons. In a river water flows alongside the shore.
I have just looked at the crash location. The water is clearly flowing alongside the shore, and it is flowing towards the west. The experienced adventurer then knows which way it is to the sea (or the larger body of water), should that prove to be of importance at some point in the story.
