Plastic pollution in our oceans is a major threat to many marine species, through ingestion, entanglement, and chemical toxins, as well as other threats.
Chemical toxins can affect the diversity of macrobenthic communities - microscopic organisms living at the bottom of the water column; ingestion of plastic has been recorded to affect reproductive rates of organisms such as the Laysan albatross; and entanglement and ensnarement has been recorded to affect at least 135 marine species - including cetaceans, sea snakes, and seabirds.
Plastic bottles may look harmless, but when they find their way into our oceans, not only do they risk ensnaring an organism, but the chemicals in the plastic disperse into the water, and the bottle itself will eventually form smaller and smaller fragments, known as microplastics.
If you’re interested in this, I’d recommend looking at Vegter A. C. at al. 2014 “Global research priorities to mitigate plastic pollution impacts on marine wildlife”, Endangered Species Research; Volume 25, pp. 225-247