To recall, sea-birds confuse small bits of plastic with food. They swallow the bits of plastic and it kills them. To recall, abandoned fishing gear entangles fish, turtles and marine mammals, and its kills them. Floating plastic bags do the same. That is not the end of the story
Each year some 6.4 million tonnes of litter are entering the oceans, or about one kilogram for each human being. Most litter stems from cities. “The world’s cities currently generate around 1.3 billion tonnes of municipal solid waste a year, or 1.2 kilogram per city-dweller per day” (Economist 7th June 2012). The litter discharged into the sea corresponds to about 4-5% of the municipal solid waste produced in the cities of the world. In that sense its a small part of waste produced by the nine billion humans, but as any dumping it causes problems.
Persistent littering the sea likely has started with disposing “clinker” from steamships and currently has found its peak with “plastic”. Clinker, the residue of burnt coal, was commonly dumped from steamships well into the 20th century. Currently, the most abundant marine litter is plastic. Plastic accumulation on the seabed is more abundant than in the open sea. On the global scale, the ocean currents sweep the litter to the centre of the ocean-gyres where it accumulates, called the big garbage patch. On the local scale, litter is washed upon the beach. On the hidden scale, litters is channelled to the deep sea. Marine litters accumulates in particular high densities in submarine canyons. Submarine canyons act as passages for litter transport from continental shelves into deeper waters. A survey  of European seas published in April 2014, confirmed again that marine litter is found everywhere, from the beach down to the deep sea. Litter density in submarine canyons reached an average of 12.2 – 6,4 items per ten-thousand square meters, or the double of the litter density found elsewhere. A litter density of 12.2 – 6,4 items per ten-thousand square meters means to find about 10 items of litter on a surface wide as a football field!
Litter is a serious risk for the marine environment. The degradation of plastics generates micro-plastics that are ingested by organisms, leading to contaminants across trophic levels up to the fish  that we may eat. So plastic debris may return to their source, finally.
 Christopher K. Pham et. Al, 2014. Marine Litter Distribution and Density in European Seas, from the Shelves to Deep Basins. PLOS ONE, 9(4), pp. 1-13.
 Chelsea M. Rochman et al. 2013, Ingested plastic transfers hazardous chemicals to fish and induces hepatic stress, Nature, doi:10.1038/srep03263;