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.
Some rivers are tidal such as the Thames and Severn to name a couple of largeish nearby examples. I'm not sure exactly how far upriver it can go but it can cause chaos in areas that are significantly inland.
