Means of signaling have been suggested.
OK a couple of commercial PFD's also.
But you don't want to be obvious. You don't want to be singled out on the beach. Who goes swiming at the beach in a lifevest. Even one that is uninflated.
You have a pocket. Why not put a couple or three 1 Gallon Zip Loc bags in your pocket. Then if you need floatation Simply inflate and zip shut you have a pillow of air.
So you can only use 2. One under each arm. Why did I suggest 3? Depending on the time you need you might lose one. Or one could get a hole in it while stored in the pocket. You are not going to remove and replace them every swim session. The third is sort of an installed spare.
Good idea, but maybe too small. In Lowe's the other day, in the section that has shelves and plastic storage bins, I saw Zip-lock bags that were being marketed as storage containers. They looked like they could hold a throw-pillow from a sofa! I'd guess they're about a 5-gal size.