Sailors, who travel the seas of Avernus, often searching for exotic fish to sell to gourmet diners and the Adeptus Mechanicus.
Now to take this to its logical conclusion.
***
The Seven Seas of Avernus
Avernus is a terrestrial world with continents and vast oceans of salty water. These oceans are, like the rest of Avernus, full of terrifyingly lethal features and wildlife which are utterly inimical to human existence and civilisation as a whole. In fact, the oceans can be considered to be even more dangerous than much of the land, so filled with dangers that Man for all its powerful technologies cannot reliably venture into it without meeting a horrible death. Its inherent danger can even be felt by looking at native Avernites - whereas a multitude of civilisations can be found on land, only one can be found that has anything to do with the seas. With humans and xenos alike keeping a wide birth away from the sea, there was no doubt that whatever potential draws it might have are outweighed by the many horrible, hungry downsides.
Or so it was thought.
It began with the Caverns. Found underneath Avernus' Spine, the caverns were (and absolutely still are) a level of terror and death far surpassing that of even regular Avernus. Despite this, it's drawn a substantial number of people into its depths for various reasons, the most common being greed, a desire for challenge, and a desire for death. From the Sirens it was known the oceans were indeed less dangerous than the caverns, so it was thought that since the caverns could be tackled so too could the seas. The initial preparations were facilitated by enthusiastic support from certain members of the tech priesthood. These members greatly desired to construct some of the wet navy designs found by the Reclaimer that never had any reason to be built.
In the beginning of this new wave of Avernus exploration, casualties were atrocious. The oceans were indeed less dangerous than the caverns, but humans were far less suited towards venturing into the ocean than the caverns. The Imperium had overflowing data and methods of waging war on the ground, in the air, and in the void, but almost nothing for wet navy warfare outside of the manuals for PDFs of certain worlds. The closest equivalent to highly-developed wet navy doctrine they had was their void navy doctrine, which the first generation of sailors weren't well versed in. Despite the initial setbacks and with the advice of those versed in void warfare, institutional experience was quickly accumulated, wet navy warfare doctrines were hashed out, and training regimens for the interested were programmed. With a firm footing established and after several decades to accumulate further knowledge, Avernus became home to a proud sailing tradition and the most lethal human fishermen in the entire galaxy.
With increasingly greater levels of skill and knowledge accumulated over the years, Avernite sailors have been able to range out further and further away from civilisation in search of rarer catches. With increasing wealth coming in from sailing, more people were enlisting to becoming crew members, ships of greater quality were being built, and things were generally on the up and up. A status quo had been reached and it was one of relatively peaceful and steady growth. On one foggy, nondescript day, however, this status quo would be altered by something no one ever really expected: xenos ships.
It's long been known that Avernus is home to xenos civilisations of many different kinds, with several already found near human settlements such as the Sirens, the Mycenids, and the Trolls. Avernus was bound to still hold many more xenos civilisations that remained undiscovered. What wasn't so easily predicted was that even with levels of technology far inferior to that of Man, land-bound native civilisations could and would still make ships to sail the seas of Avernus. It thus came as something of a shock when a ship in the process of returning a catch of acid-slitted eels crashed into a strange xenos-crewed ship that was hidden by the unusually strong fog that day. In the ensuing panic, battle broke out and both sides quickly took a mauling; the humans mostly in the form of lost crew and the xenos mostly in the form of damage to their ship. Both ships hastily withdrew in the wake of the damage they were each taking, leaving the fishing ship to limp back into port and report what happened.
Years passed and between exploratory expeditions, psykers, and the efforts of the Ordo Dialogous, a new maritime world was opened up for the humans of Avernus.
***
The Peoples of Avernus have been sailors for millennia. Whether it be in vessels of wood, metal, coral, bone, living flesh, or even more exotic materials, xenos of many different races and civilisations have plied the ocean blue for much of their recorded history.
The two main reasons for seafaring in regards to members of xenos civilisations are fishing and travel. It's easy to understand why they fish as they're the same as humans in this regard, but a little less intuitive for humans to understand is why they use it to travel, seeing as the sea is typically more dangerous than land. At first it was theorised that elsewhere the land was more dangerous than the sea, but in fact the standard holds true in other places as well. While the sea is indeed more dangerous than the land, it's also much faster to travel from one place to another by sea than by land. In terms of net total danger to be found, there is less when travelling by sea than by land, even if the average danger you'll encounter per day will be greater. (Humans have never needed to worry about this because they can transport cargo and people by air and rail, so seafaring for travel is obsolete.)
Warships are used by xenos for mainly two things: war (as one might expect) and pirate-hunting. (Combating wildlife is too cost-ineffective for them; that job is left for the fishers.) Warships are used in numbers and in brief bursts. Whether it be for raids, invasions, or taking out enemy ships, a large mass of warships is required to do the job right and reliably. Large concentrations of ships do attract particular large and moderately undefeatable predators, however, so whatever actions a given navy wants to do have to be completed quickly.
While seafaring civilians and warriors comprise the greater majority of sailors belonging to Avernite civilisations, they cannot in truth be called the most noteworthy of their sailors, even collectively. It's their corsairs that are most worthy of mention.
Avernus is a world full of danger, death, and insanity waiting at every corner, but it's also a world of riches waiting to be seized by those with the means and will to see it through. Corsairs are those rare adventurers who go out into the particularly dreadful regions of Avernus that are its oceans. In simplest terms, Avernite corsairs are a combination of pirate, explorer, and adventurer, similar in ways to the rogue traders of the Imperium though even less beholden to governments. The story of a ship travelling to exotic new lands, finding powerful weapons in the ruins of an ancient civilisation, and returning with the lost hoard of a doomed corsair is just one of many tales that drive so many to pursue the lifestyle despite the immense risks involved. Fame, power, and wealth beyond one's wildest dreams are the prizes that can be won by corsairs and their crews.
The most powerful corsairs are the Corsair Lords. Corsair Lords are those corsairs that through various means have acquired at least one of the eight Golden Relics of the Corsair King - a legendary corsair who lived thousands of years ago and who some say was the very first corsair. The eight Golden Relics are golden artifacts of various kinds that have various magical properties and are seemingly indestructible. Legend says that when brought together, they will reveal a map that leads to the treasure of the Corsair King. So far, only six of the Golden Relics have been found, the others presumably still hidden somewhere on Avernus.
While all corsairs are characterised by a burning ambition that compels them to venture out into the blood-soaked seas of Avernus in pursuit of their goals, they come from many races and can vary greatly in background, motivation, and ethos. Thanks to the expeditions undertaken by humans and the resulting exchanges of knowledge, information, and stories, humans can now be found as the crews, officers, and captains of their own corsair ships. The officers of a corsair ship are for the most part the same or similar but in some ways they're quite different.
Captain: The leader of corsair crew themselves, the captain holds supreme authority on board their vessel. Leadership qualities are key to their success.
Quartermaster: The one in charge of supplies, money, and the ordinary crew. Typically the second in command.
Navigator: Navigators are in charge of a ship's sensorium, managing its charts and maps, setting the ship's course, and steering the ship.
Chaplain: A member of the Adeptus Ministorum. Typically this role is filled by a Preacher, but Sisters of the Orders Dialogous or Hospitaller have also been known to serve in this role.
Explorator: A member of the Adeptus Mechanicus. This Magos is in charge of the tech priest contingent of a ship.
Psyker: The seas of Avernus are a place of psychic death. It's thus very worthwhile for a corsair to bargain with the Adeptus Astra Telepathica for a psyker to be attached to their ship.
The most renowned human corsair is a man named Gades Helix. His biggest claim to fame is defeating a powerful Sea Dragon in a weeks-long running battle that required a nigh-miraculous level of performance from the entirety of the ship's crew. The battle ended when Helix took advantage of the Sea Dragon's exhausted and heavily injured state to board a Valkyrie and charge it with power axe in hand, decapitating it in a single blow. Using some of the creature's diluted blood for promethium, the badly damaged and undermanned ship managed to limp back to human waters towing the corpse of the Sea Dragon behind it. Helix sold its body for an immensely high price to the Mechanicus, had one of its eyelids fashioned into a cloak for himself, and mounted its head on the prow of his ship. Later on his ludicrous wealth would be supplemented even further by the bounties a number of xenos civilisations had apparently placed on the creature.
***
Tricorn hats can be found worn by the occasional xenos corsair. It's a bit strange given that in the entire galaxy only orks and humans have been known to wear such hats, but it's probably just a coincidence.
