The invention concerns a method and an apparatus (10) for treatment of water (26) to be injected into a subsea injection well, the apparatus (10) being disposed in water overlying the well, preferably on a water bed (12). The invention is characterized in that the apparatus (10) contains at least one receptacle (38, 40, 46, 48, 50) provided with at least one water-soluble solid-state chemical (42, 52, 54, 56) for treatment of the injection water (26). The water (26) is brought into contact with the at least one chemical (42, 52, 54, 56), causing it to gradually dissolve and mix with the water (26), after which treated water (26') is led into an injection stream to the injection well.