Under perfect conditions, we can see the Andromeda galaxy, 2.5 million lightyears away, with the naked eye. Underwater, great visibility is measured in meters and feet. Think about that for a few minutes. Try to wrap your brain around it.
Photos courtesy of waitbutwhy.com. Normally, Andromeda looks like a fuzzy star. These images were enhanced to show what it would like if all its stars were visible. Our moon is used for a size comparison.
How did I find this? The question was: how do we have better maps of our universe than of our oceans? We have 100-meter maps of the surface of Venus, but 5-km maps of our oceans.
The answer is that we can see through space and atmospheres. We can barely see through water, even with our most advanced scientific instruments. We can see galaxies billions of lightyears away with our most advanced telescopes, yet we get excited when we can see 30 meters (100 feet) with a dive mask.
Seriously, my brain was hurting for a while after this revelation….
Plus, look again at how Andromeda would look in the night sky if all its stars were visible. It takes its light 2.5 million years to reach us, and it looks absolutely massive.