That doesn't appear to pass the smell test. According to this* there are only 52,000 merchant ships operating globally so the thought of each ship dumping 20,000lbs of plastic overboard each year in acerage (a rough estimate based on 5.6m tons currently existing and that number "regenerating" each decade as old plastic falls to the sea floor or gets ingested) seems to be unreasonable. I've seen some of the rivers referenced in that "forwards fron grandma" level infographic and I'm inclined to believe it.