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Salty Cells
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The bodies of all fish are made of cells. The liquid inside the cells of fish contain some salt. The cell solution is less salty than sea water, but more salty than fresh water.
Permeable
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The cell walls of all fish are made of membranes, which are permeable to water molecules.
Osmosis
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When a permeable membrane separates solutions with different concentrations of salt, water molecules from the less salty solution will diffuse through the membrane into the solution that is more salty in a process called osmosis.
Saltwater Fish
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Osmosis causes the cells in saltwater fish to constantly lose water, which the fish has to replace by drinking salt water, desalinating it with special organs and replenishing the water in its cells.
Freshwater Fish
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Osmosis causes the cells in freshwater fish to constantly absorb water so freshwater fish never "drink." But they do have organs that remove the excess water, which is excreted similarly to how mammals excrete urine.
Changing Environments
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A freshwater fish put in salt water would quickly lose water from its cells, with no way for it to be replaced, and perish.
The cells in a saltwater fish put in freshwater would quickly absorb water from the environment, and with no way to eliminate it, the cells would swell, burst and kill the fish.
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Freshwater & Saltwater Fish
Thousands of species of fish live in freshwater environments, and even more thousands of species occur only in saltwater environments. Other than in relatively rare instances, most freshwater fish can't live in salt water, and most saltwater fish can't survive in freshwater.