I very much enjoyed the quality of prose and the tone of this piece. I really found his dissent well characterized, how he started to enjoy suffering and his use of pain to try and track time. I enjoyed reading it, found it engaging and thank you for that. If I had written this I would be proud.
I think as an RPG supplement this is wonderful, players could take this information and discuss "shell hell" as a down side to water travel, wear little turtle offerings totems or maybe even seek to enter shell hell in order to get information from a dwarf damned there.
The questions regarding what the demon wants or his ethos or origin or his deity social circle should he have one are irrelevant from an emotional roleplaying perspective-this is the boogyman of the ocean. But some discussion of those might enrich this as plot device if a gaming group ever needs to deal with Nuverl. For example do you in vision Nuverl as being present ii all water as seen by the dwarves or just sea water? Perhaps when dwarves take baths they wear little turtle totems. Perhaps this dwarven fear of Nuverl extends to all pooled water, and they may touch their totems before picking up a bucket or dipping their hands into a finger bowl (assuming your dwarves have finger bowls).
Some thoughts about the world building and the story telling. These are not criticism but things I might offer where this discussed in a story workshop.
World Building: I like the tone. It is impersonal and it catalogs the dwarves suffering in manner almost devoid of self piety. It has a Lovecraftian feel to it. But if you were to change the journal entries to more personal and self reflective you could transmit more information about the dwarf's world as well as his plight.
For example, perhaps his desirous thirst manifests as some pontification about standing in line at his favorite distillers back home. Perhaps instead of unaddressed journal entries he direct his writing to his son, wife or father and in this way the character discusses his relationship with them. Right now there is very little "dwarf" (no pun) in the story. You have a bit of this in the 45/46 entry. Perhaps you could add more. Of course adding this info. may change the tone, which I think is very good right now, so take that worth a drop of water.
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Storytelling: I would add a climax to the story. Perhaps he finds an exit and almost gets out or feels trapped by on of the other damned creatures and tries to communicate with it. Perhaps he tries to eat one of the other damned dwarves. The climax could only be in the mind of the character in which he thinks something is going to change and he is going to escape or figure it out.