Sea water fish are fish that live in the sea, as opposed to freshwater species, and the term covers the great commercial species: cod, haddock, halibut, herring, mackerel, tuna, and thousands more. Life in salt water shapes their biology: they constantly lose water to the salty environment and must drink seawater, excreting the excess salt through gills and kidneys. For the cook, sea fish offer flavours from the delicate to the robust, and their flesh, often leaner than freshwater fish, suits quick cooking. Sourcing matters enormously, since many sea fisheries are overexploited; certified sustainable choices and seasonal awareness help. Salt content of the flesh is barely higher than freshwater fish: the sea seasons the life, not the fillet.