Hi Sharon.L, and thanks for your post. While there is a device that is commercially available, called "oxyfloat", manufactured by 'oxyfit', I would caution you about using a portable oxygen concentrator (POC) with this device. From what I've read and know, the device, oxyfloat, is designed for the use of a single small-sized oxygen cylinder.
If you were to use a POC, and it were to fall or be overrun (for any reason) with water, the POC (as an electric device) would most certainly become damaged. The manufacturer makes no mention of using POC's with the oxyfloat.
On the other hand, if a cylinder were to get wet or become inundated with water. there would be no permanent harm to the cylinder.
Naturally, the decision is yours.
When using oxygen while swimming, safety becomes of paramount importance. I urge you to evaluate what you feel will work best for you and still allows you to participate in pool activities.
Good luck!
Leon (site moderator)