From what I understand your phone was sold as being Water Resistant not Water Proof.
While these are very similar terms they are different enough when it comes in the construction of the phone. A water proof device would require gaskets and a sealer preventing water to enter at a depth of immersion where as a water resistant device would only need to make the openings as small as possible so water would not have entry without immersion.
Lifting the glass off and then reseating it using the original glue will not be as water tight as it was when it was made. You will likely not be able to achieve the same level of sealing as it was new even if did clean and apply new glue.