There are a few factors that make it almost impossible to make the repair as watertight as it was. This also includes Apple's own repairs!
First the surfaces where the glass is attached need to be perfectly clean and not have any dings or warpage. The adhesive needs to be applied just so and then the glass surface the adhesive attaches to needs to be spotless. Even then you need a press to hold the glass to the frame so the pressure is even and held for a good 10 hours so the bonding of the adhesive is complete.
If you look at the watch makers or companies that make underwater housings they use a very different method of sealing! They use O rings so the seal is compressive. My cheap Seiko watch has been down over 60 ft SCUBA diving I wouldn't dare do that with my Apple Watch or iPhone new or serviced!