Ghost touch issues can have several causes. The most expensive to fix is if there's an issue with the touch control IC or the connection to the logic board. Apple doesn't specifically fix that. There could be a bad connection with the ribbon connector, and that fix can even be free. But digitizers are the most common reason and are of fixed with a complete screen assembly replacement.
If it's the digitizer, then I'd think the water resistance would be as good as new. More than likely the resistance has been decreased over time with normal wear. A new screen assembly installation by Apple or an AASP would require use of a new set of water/dust resistant seals.
When Apple gave a Reuters reporter and photographer access to their repair development lab, they showed them the equipment used for repairs. That includes new seals (there's a photo of them), a press that "activates" (I believe using heat) the seals as it presses the screen assembly in place, and their Horizon Machine that does a bunch of mechanical integrity tests as well as calibration and pairing. The machine is needed to get the biometric systems to work with the processor.
https://www.reuters.com/article/us-apple-repair-exclusive/exclusive-apple-makes-iphone-screen-fixes-easier-as-states-mull-repair-laws-idUSKBN18Y0BF
Here are some of the captions from the slideshow:
An Apple technician uses a press with an integrated timer to activate adhesives on an iPhone at the company's display repair laboratory in Sunnyvale, California, U.S., May 19, 2017.
A sheet of display assembly adhesive is seen at the Apple display repair laboratory in Sunnyvale, California, U.S., May 19, 2017.
Apologies as I'm adding more stuff via edits.
