In case you didn't know, osmosis is the flow of solvent (here its water) across a semipermeable membrane from lower concentration of solutes to a higher concentration.
Freshwater fishes have their bodies with body fluids isotonic, or same concentration, with the surrounding freshwater body. If you throw such a fish into seawater, considering that seawater has a lot more solutes dissolved in it, water will flow from the fish into the seawater. The fish would get deprived of water and will most probably die.
Note: Certain fishes like Sockeye Salmon can survive both saltwater and freshwater. Sockeye in particular usually lives in marine habitats, but returns to freshwater for spawning (egg laying). This fish overcomes the osmotic problem, by staying in the intertidal zone for a while, thus adjusting it's body fluids to the intertidal zone, and then finally moves to the freshwater. Yet, some others have non-permeable membranes as skins, so that osmosis isn't a problem for them.