By making a small tunnel on bottom of the ocean and lighting up the floor, when I swam through it, water was not the same as before. When I reached the top and bottom of a tunnel (with water inside), flowing through it, I had standard visibility. However, when I got to the middle of the height of the tunnel and was swimming at that level, there was increased darkness/ visibility increased. With respiration I, II, and III enchantments, the difference is less noticeable but is still present. Besides that, the colors of the blocks underwater were more saturated while swimming in the middle of the tunnel.
To reproduce:
1. Create a tunnel underwater (don't remove water from tunnel).
2. Light up the bottom of tunnel with glowstone.
3. Swim at the bottom of the tunnel, then the middle, then the top.
4. You should be able to see the visual difference.
5. You may repeat with respiration enchantment to see the difference more clearly