Phuket beaches are actually dangerous close to shore when breakers are very strong (red flag conditions,which so many people just ignore). Very uneven bottom and people trying to walk to shore are being pummeled off their feet by big breakers from behind whilst in quite shallow water. Sea and sand create a soup meaning you don't know which way is up until the following wave is already coming in, meantime the strong ebb of the last wave is sucking you away from shore and the sand bottom is like quicksand as it's all in motion. Finding your feet on almost liquid sand can be very difficult, and body strength is the important thing, not swimming ability.
RIP to the several swimmers who have already died in the sea at Phuket this year.