I have not purchased or used the back glass for the X series so hopefully someone else will chime in.
As for waterproofness…the iPhone XS is water resistant, not water proof. See this support page by Apple for more details.
iPhone XS and iPhone XS Max have a rating of IP68 under IEC standard 60529 (maximum depth of 2 meters up to 30 minutes). iPhone XR, iPhone X, iPhone 8, iPhone 8 Plus, iPhone 7, and iPhone 7 Plus have a rating of IP67 under IEC standard 60529 (maximum depth of 1 meter up to 30 minutes). Splash, water, and dust resistance are not permanent conditions and resistance might decrease as a result of normal wear. Liquid damage is not covered under warranty.
Now, IMHO, these specs are only attainable when the phone is assembled in factory conditions (dust-free environment, repeatable processes etc.). Any repair outside of those conditions will not achieve the same level of water resistance. It may, but statistically speaking, any device you or any other third party shop does will not meet those specifications.