Frederick Marryat (who was one) or Patrick O’Brian?
Marryat describes filthy living conditions, brawls, drunkenness and whoring in a Flashmanesqe manner.
O’Brian describes more of a kindly tutelage
with the frequent maiming or death.
I think POB has plenty of filthy living conditions, brawls, drunkenness and whoring for his mids.
Filthy living conditions - pretty much in every book.
brawls - Babbington fought another mid over a chest for calling the Doctor a “Dutch-built bugger”.
Drunkenness - they got the newest mid drunk in “Desolation Island”, leading to him telling Jack “Do it yourself Goldilocks” when being ordered to fetch a glass.
Whoring - Pretty much every mention of Babbington up until he marries Admiral Harte’s daughter. When Jack sends him ashore in Bombay to find the Doctor (after Babbington has memorized the phrase “Woman, wilt thou lay with me?” in multiple languages), he makes him turn out his pockets first to make sure he doesn’t have enough money to go to a brothel.
You have to remember too that Jack was pretty much one of the best officers in the service - certainly we see plenty of officers who treat their crew much worse. And while Jack took his duties of turning mids into officers seriously (that’s how you keep the Navy healthy after all), he certainly wasn’t above beating his mids. Poor Fanshaw got seized to a gun and whipped for calling Abraham “No one of any consequence - just another wicked old Jew.”
There’s a passage in one of the O’Brian books that’s always stuck in my mind, in which Maturin casually comments that his cabin was unusually dry and cozy, since he could only sink his finger into the mold growing on the wall up to his second knuckle.
But that was the Horrible Old Leopard, was it not? 
That was the Worcester (I happen to have The Ionian Mission on hand), and actually Stephen is complaining about the mold:
“I do not like to sound discontented, Tom, but surely this ship is more than usually damp, confined, awkward, comfortless? The mould on the beam that traverses my cabin is two inches thick, and although I am no Goliath my head beats against it.”
Also, I don’t really recall Marryat’s descriptions of drunkedness and whoring. I thought Marryat’s descriptions were much more romanticized views of life on board ship than O’Brian’s.
I’m currently reading Frank Mildmay. Perhaps he is more of a rake and a cad than Marryat’s other characters.
I guess there’s less whoring among Jack’s midshipmen than usual since Jack (rather hypocritically) won’t allow women on board. But Stephen often complains about the noise and the smell that comes from the midshipmen’s berth. And they’re often hungry, reduced to eating rats.
Then there’s Hornblower’s experience as a midshipman, where one older officer was bullying and brutalizing the rest.
O’Brian is brutal about killing off sweet, excitable young midshipmen. I’ve just been listening to Fortune of War, where he kills off Young Forshaw, “who is far prettier than his sisters, though no doubt adolescence will soon cope with that”. (Jack didn’t beat Forshaw for maligning Abraham, exactly, but for possibly “making game” of his captain.)
I’ve never read Marryat - I should.
Ah, the unreliability of memory. In my defense, it’s been close to a decade since I read that.
Probably less than a year for me, and I got it wrong, too.
To be honest, I read it a day or 2 ago, as part of my biannual reread of all the books.