A freshwater plant placed in saltwater will die because of how its environment affects the process of osmosis. The saltwater causes the freshwater within the plant to flow out, causing dehydration.
Normally, freshwater plant cells use the process of osmosis to take in or give up water. However, when the surrounding water is filled with salt, the water inside the cell moves from where there is more water to where there is less, namely the salty environment outside of the cell. This dehydrates the cells despite the fact that they're surrounded by water and results in dehydration. In extremely saline settings, this can even result in plasmolysis, or the shrinking of the protoplasm away from the cell wall, and create a gap between the wall and membrane. Because of this, freshwater plants should not be used in saltwater aquariums.