Not suitable for my purpose
I inserted the inspection hatch into my drinking water tank to allow cleaning from the inside. Unfortunately the lid is not tight. If the drinking water tank is well filled, a significant amount of water can penetrate through the lid seal when the boat rolls. When I complained over the phone, I was told that the cover was not suitable for this because it only had two points where the cover mechanically grips under the installation flange - there should be at least three if the cover is supposed to seal from the inside.
In my opinion, this restriction is not clear in the product description - I used the lid as described, namely as an inspection opening.