Really interesting idea of applying diving deeper in multiple meanings of the expression!
I liked the exploration aspect of it, although I was a bit confused. On the last character it felt like the notes were their thoughts, but before it I was under the impression that I was finding notes from other people, that wasn't very clear.
The visuals are pretty nice, although the vinette blinks when I click to advance dialogs, it feels like a glitchy thing. I felt the vignette effect for the underwater sections could be toned down a little, but they serve the purpose.
I really missed some music, gets a bit boring at times.
Also regarding dialogues, sometimes I went back to the files scene and they were still showing up on the screen. Maybe an automatic timer for them could be helpful. Since I didn't click sometimes while diving it got a bit out of sync.
One note said the character was bored so he was writing, and then said he was trapped so he was writing. Is that intentional or it was some sort of typo?
A minor thing, Itch.io's full screen is clashing with Unity's full screen button.
Overall a pretty solid story entry. I like how it's told in bits.