Personally, I wouldn’t leave my phone or purse on the beach, even hidden under a towel. Most of the time nothing will happen, but it only takes one walk-by and your whole day is ruined.
Thailand is generally safe, but beaches are public and people do watch who goes into the water and for how long. Towels and sunbeds don’t really hide anything.
The waterproof phone pouches do actually work if you get a decent one. I’ve used them swimming and snorkeling with no issues. Just test it in the sink first with some paper inside. That way at least your phone and some cash stay with you.
Another option is only bringing the absolute minimum: a bit of cash, maybe one card, and leaving the rest locked in your room. If I’m solo, I always err on the side of carrying it with me or not bringing it at all.
Better slightly inconvenienced than stressed the whole swim
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