When looking through a window with air on both sides, your view path doesn't bend.

Now with water on the other side of a conventional mask, your center view angle doesn't bend... But all of the off-axis rays bend... that's called REFRACTION. The further off-axis the view, the more the light ray bends. This is why objects look bigger and closer under water.

When a camera lens inside an underwater housing is at the center of a dome, every direction the lens sees is perpendicular to the dome wall (not off-axis). Above water a dome has no optical power.

But, underwater with air trapped inside, this shape behaves like a negative power lens and shifts the focus. Why is that?.

Remember all the negative lenses have a thin center and thick edge, just like this concave / plano lens.