Sign up to marine pollution convention
There is a global treaty called Marpol (the International Convention for the Prevention of Pollution from Ships, 1973, modified in 1978), dedicated to lessening and, if possible, eliminating dumping of garbage by ships at sea. Among other things, it bans dumping any plastic anywhere at sea. Nearly every country in the world that has a coastline has signed it.
The only exceptions are Costa Rica, Taiwan and Thailand.
Why does Thailand not go along with such a reasonable anti-pollution treaty? Perhaps it's because it enables Thai fishermen to continue to dump garbage offshore with little or no worry about legal repercussions. Of course, there are repercussions - but those affect the quality of the sea habitat and the shorelines. It appears fishermen (and Thai officialdom) don't appear to be much concerned about such things.