Apple Watch is intended for swimming and rowing, and unfortunately, this case appears unsuitable for use in water…
Coolblue digital customer service says:
Good question! A screen protector like the PanzerGlass Full Body for the Apple Watch 42mm is usually intended to protect your screen from scratches and bumps. Most screen protectors aren't completely waterproof. Therefore, water can get between the protector and the screen, especially when swimming or showering with it.
This doesn't necessarily mean you have a defective one. It's normal for water to get between many screen protectors because they don't seal airtight. If you really want waterproof protection for your Apple Watch, you usually need a special waterproof case.
In summary:
Water between the protector and the screen is normal with these types of products.
The screen protector isn't designed to keep water out.