Industrialization, development of tourism, trade and sea traffic, the perpetual increase in population, the urbanization of coastal areas as well as excessive exploitation of maritime resources have resulted in several types of pollution damaging both the sea and the marine environment. This graduation paper is focused on main sea pollutants and their harmful effect on marine ecosystems, on human health, and on the economy. The first part of the thesis outlines possible sea pollutants and categories them into either land, air or vessel pollutants. The focal point of the paper is pollution caused by vessels which is the most common (but not the biggest) source of pollution. The following topics have been analysed in detail; ballast water pollution (carrying of invasive and pathogen species), bunker and oil pollution (release of oil from vessels, tanker accidents), waste pollution (e.g. plastic and scraps of food), anti-fouling paints (release of biocides), noise (vessel engines or underwater operations etc.) and chemical substances and sewage waters. Furthermore, several current procedures for sea pollution prevention and disposal methods for such pollution are described. An overview of the most important international conventions and regulations is presented, as well as possible solutions that prevent and lessen the effects of different types of pollution and help preserve sea quality.