That demons trapped under the ocean stuff is a new one to me.
The fish who lives deep under water cannot go to the surface because they are built to live under high pressure with few O2. Their way to get food are all build on where they live. They would not survive at the surface. The fish deep down are strange built because that's the only way to survive.
The fish at the surface cannot live deeper, because they are built to survive with less pressure than can be found deeper in the ocean and need more O2 than can be found deep. There's no 'barrier'; it's they way they are built that creates the 'barrier'.
We humans cannot live in water, because we simply are not built for it. Nor can we live high up the sky, because we cannot handle the cold, we cannot get food, we cannot breath there. Same goes for the fish.
Demons in legends, myths and the Bible are banned to the depts of the ocean because humans knows less about the bottom of the ocean than the moon or any other planet in our solarsystem. It's something unknown: and what is the best place to ban something we are afraid of to somewhere we do not know about?
I live on a sailboat and was wondering if saltwater is a passive counter meassure. Salt is said to be and running water as well. I have had some very wonderful OBEs on my boat so could there be valitity to this assumption. Traditionally the "deepest parts of the Ocean" had been the place of banishment for Demons and rebellious spirits in Christian exorsism for the last 2000 years. I read that there is sort of a barrier somewhere down there hindering the fishes above to go lower and the ones down there to come up. They all look very wierd is it possible that they are possesed by these demons and in a sort of prison or enclave there