You asked, "From my perspective, does my X-ray vision cause day light from outside to pass through the upper deck into this lower deck, illuminating the space as if it were day light?"
A reasonable interpretation is:
From your perspective, your X-ray vision causes light from outside to pass through the upper deck into this lower deck, illuminating the space as if it were light.
See below for the fine points.
X-Ray is a Metaphor
A reasonable assumption is that "x-ray" is being used as a metaphor, as it is in the real world. In the real world, x-ray vision doesn't mean x-rays in the medical sense, it means being able to see through things, and the exact meaning changes with the narrative. See https://en.wikipedia.org/wiki/X-ray_vision.
Transparent has a Lot of Meanings
The word transparent means you can see through something. In normal everyday speech, it is used without precision. It's reasonable to say that glass is transparent. Obviously, it isn't perfectly transparent, but it's reasonable to say that glass is transparent, meaning you can still see the glass (usually), but you can also see through the glass.
What You See
Taking the description as much on face value as possible, a reasonable interpretation is:
Within 30 feet you see everything, except as blocked as described. You see the treasure chest, and the stuff inside. You can look at a book and see all the pages on top of each other. You see the print facing you normally, and you see the print facing away from you reversed. You can look at a companion and see their beating heart, and the blood that beats through it. You just regularly see it all at the same time. In all cases you see in color, and you see the different parts of the object at the same time.
It doesn't make any real-world sense, but hey, magic.
Light isn't supplied
Inside a pirate ship it would be dark.
Unless there were a light source, and then it wouldn't be dark.
Or unless your 30-foot x-ray sphere intersects with the deck, and it's daytime, in which case to you alone it would appear as if light were passing through the intersection of sphere and deck and lighting up everything in its path. This is the scenario you described, I think.
If you have a trait that lets you see in the dark, such as darkvision, then you can see as described by the feature.
So, if there's light, or if you have the ability to see in the dark, then you can see; otherwise, it's dark.
Another Thought Experiment
Perhaps another useful thought experiment is to imagine you are standing in a fantasy apartment building assumed to have no stone, metal, or wood such as would block vision as described in the item description, completely un-illuminated, and you are more than 30 feet from any outer surface.
You can't see a thing, it's dark.
You turn on your fantasy flashlight; you can now see out to 30 feet as if the light could penetrate all objects. Where the flashlight is directly shining, you see everything and through everything as if normally illuminated by the flashlight; out of the main beam you see everything and through everything as normally illuminated by a flashlight out of the main beam.
You turn off the light and step to slightly less than 30 feet from an outside wall, such that the 30-foot x-ray sphere and the outside wall intersect in a 6-inch circle; it is daylight outside. You now see within the 30-foot sphere as if there were a 6-inch porthole in the side of the circle.
An interesting question is, does light appear to attenuate to you? In other words, in the real-world, the light of an object 30 feet away receives less light than a same-sized object 10-feet away from a flashlight, therefore the closer object is more brightly lit. Does the same hold true for you? Again, a lot of room for interpretation, but it is reasonable that where the rules say a torch illuminates to 10 feet, that is not negated by the x-ray vision; but on the other hand, daylight does not come with a radius so that light would illuminate to the other side of the sphere.
Another interesting question is, how does blocking work? In the fantasy apartment building, assume it is built of wood. To my reading, where from your point of view, there is a continuous 3 feet of wood, your vision is blocked there. You can see through a wood wall. Stand on top of the wall such that you are attempting to see through 3 feet of solid wood, it's blocked. Same with stone and metal as described. I don't see it as cumulative. Layers of metal half an inch thick stacked up would not block the vision so long as there was air or non-metal between the layers. How much material between the layers? I don't know, but you might or might not be able to research an answer in your downtime.
Only Light, and Only You
Perhaps obviously, only you "see the light". It's magic, not physics. No one else sees the light. If daylight shines on the deck, only light comes through. If you're a vampire, you are only affected as if you were standing in light, not daylight.
What is Reasonable
What is reasonable is subject to interpretation. Many games I have played in write down agreed-upon interpretations as rulings. This might be a good case for such a practice.