Glad to hear that this isn't normal. I remember reading something about excessive plastic trash on Phu Quoc beaches (on TA? Wikitravel?).

I've certainly had bad luck with dirty beaches elsewhere -- The Andaman during extreme high tide and Sheraton Senggigi after a storm come to mind -- but PQC was the most disgusting I ever saw. Below are pictures of the ocean with trash trash floating, plus the bulldozer that hauled away what a crew of 10-12 people had raked up in an effort that took all morning. There were piles like the one pictured every few feet, and the bulldozer made a dozen+ trips to remove it all.

I will definitely keep an eye on the weather patterns when I come here again.