A bag left near our stuff, descretley hidden and my eyes had to stay on it the whole time.
I didnt enjoy having to nanny my stuff and constantly worry about it, so I tried a floating toy with a water proof container that housed our drinks (and my pistol). Its large enough to not lose sight of and my pistol stays dry and accessible.
when I've gone to the river I used to put my ID and keys in a zip lock, when I put our beach town down to claim our spot after bit I would descreetly bury the ID and keys in the sand under the beach towel. The idea here is that if your off swimming, or otherwise away from your carry bag/backpack and someone took advantage of that to grab and run off with it... I would still be able to drive home. The same trick could work with a small pistol.