You’re got the “Hydration” thing right Mark @Banting.
And hydration is not only needed for proper operation of a CGM sensor but also for decent management of our body glucose levels -BGL. As a body dries out - dehydration, the interstitial fluids [that watery stuff in which our body cells float] diminish; the CGM works by analyzing glucose in these fluids [NOT in BLOOD] so during dehydration readings could be inaccurate or non-existent.
Dehydration is often the first ‘problem’ noticed in patients who are brought to hospital emergency with high BG readings. That is why the first treatment in those facilities is to begin IV hydration therapy.