Phuket's beaches, canals and water supplies are different systems. A sample from one spot can't clear the whole island.
Check the beach name, the date, and what was tested. A coastal sample is just one spot, at one time. It can't speak for another beach. It can't speak for tomorrow either.
Rain, runoff, boats and building work can change coastal water fast. Keep the date next to every result.
Thailand's Pollution Control Department (PCD) publishes surface-water data. Use it for background. Follow any current local swimming notice too — this data is not a live safety check.
Ask your hotel or water provider about treatment and current advice. A beach result says nothing about tap-water safety.