Can you elaborate on this? Why can't there be things that exist but aren't mentioned in the Bible?
One of the assertions of the HPT is that there is nothing miraculous about the flood or creation, other than what is stated explicitly or is obviously a miracle, in scripture. For example, God creating things is clearly a miracle, but the animals boarding the ark, while they would be under God's influence, didn't magically/miraculously appear on the ark, they boarded it with the help of Noah and his family.
A canopy of water above the atmosphere would have to be miraculously put there and sustained there, because there's no physical way for a sphere of water to remain above the atmosphere without it falling to the earth, not to mention that it would instantly, simultaneously freeze and boil, and so would have required God to miraculously keep it a liquid for it to become rain.