@romullus , thanks for your answer. Yep, it looks like a serious flaw...
Obiously, the closest the cam is from the water line, the less the box is visible. But if you want to see the water waves, you have to move the cam up.
It's extremely easy to replicate: Create a CoronaCam, set a Size in"Camera physical size" (tick Show in Viewport), and move the Target.
I was waiting for an answer from the Corona Team, like, is there any workaround? Will it be fixed in a future release? Or is it the expected Camera behaviour?
Thank you