How dangerous is plastic pollution in our seas?

There has been some confusion over the level of environmental damage that discarded plastics do to our environment. These links show some of the research that has been going on in the field of marine plastics pollution:

http://news.bbc.co.uk/1/hi/sci/tech/7316441.stm

http://news.bbc.co.uk/1/hi/sci/tech/6218698.stm

British beaches are littered with plastic:

http://news.bbc.co.uk/1/hi/uk/3672091.stm

And so are those of Hawaii:

http://news.bbc.co.uk/1/hi/world/americas/6740161.stm

Now many countries are banning flimsy plastic bags in an effort to cut down on waste and protect resources:

http://news.bbc.co.uk/1/hi/world/7268960.stm