Science has found that there is 6 times more plastic by weight in some areas of the ocean than naturally occuring plankton. There is a lack of investigation on the effects on hormone disruption and the endocrine system of marine life. If there are disruptions in the reproductive cycles of certain fish for instance, this could be a devastating extra pressure on economically important fish species that are already being overfished.
So...consider the implications each time you take a plastic bag at the shop....
where does it end up .. why are we using plastic packing that lasts for decades to transport things we're going to eat within days ??
And here's a nice educational website for children on keeping the oceans clean.