Continuous glucose monitors and showering

I assume you mean the Libre.
These stick pretty firmly in the shower.
You may need to add extra tape for prolonged immersion such as when swimming.

Libre cannot be removed and put back on, it lasts for 14 days.

Dexcom G6 has a 10 day life.

Sticks fine even when sweaty or showering. Not removable to shower and so on, but comes off easily when it expires.
Also, no skin reaction with the adhesive that I suffered with using Libre.

Dearer than Libre, at £160 a month on subscription, but very accurate when compared to finger pricking. I trust it over the G5 and the Libre.