Plastic enters the ocean via...

...waste disposal from merchant and fishing vessels, offshore dumping, accidental or deliberate discharge of the raw pellets used by the plastics industry (Joyner & Frew 1991), material left behind by beachgoers, and waste carried into the oceans by rivers and drainage systems (Pruter 1987, Williams & Simmons 1997). Plastic is found in the surface waters of all of the world’s oceans and poses a potential hazard to much marine life, including seabirds through entanglment or ingestion (Laist 1987).

excerpted from:
http://www.marineornithology.org/PDF/32_2/32_2_187-189.pdf