There are lots of ways devious authors ( 🙂 ) try to disguise exposition in the openings of their novels. The “story within a story” structure is one. Here is a good example. This opening is well-written, but is it the story’s natural beginning? I’m not sure.

Title of the work: The Lost Provider
Wordcount: 92000
Genre: Fantasy
Language: US English
Synopsis: A critical scholar pursues tales of a rebellion and a folk hero across a foreign land trampled between two empires on the suspicion that he might find answers about his missing father. He finds himself in a swashbuckling adventure he is woefully unprepared for.
Text:
“I’ll tell you all how the Rebellion started. The Larbant enforcers had built this prison, the most fortified they could muster, where they locked up all―all the strongest, toughest, meanest Bors. T’were the captain’s first mistake. His second mistake were he chose his most veteran guards. He posted ten inside the prison, aye, and ten more outside. Ex-soldiers, these were, weathered and tried. They could handle their prisoners easy. Yet they shifted uneasy at their posts while midnight drew close in the midst of the dark moon.”

“Why not throw some thunder and lightning in there too?” called a burly member of the impromptu audience.

“Aye, good call,” agreed the storyteller from atop the table he had commandeered for use as a stage. He was the sort commonly described as a salt-encrusted barnacle, a wiry fellow prematurely aged by the sun, wind, and sea. He also already had two sheets to the wind and working on his third with the mug in his non-gesticulating hand.

The alehouse sported a line of windows facing out to sea through which the red sky shone. The raucous interior was stuffed with seamen of all stripes, and many carried on their own blusterous conversations, paying the storyteller no mind. Able Houser, however, had turned away from the deckhands he’d been talking with to better listen.

“A thunder rolled in the distance,” the old seadog was loud enough to contend with the hubbub, “but the Captain thought himself too wise to trust the hair rising on the back of his neck and too important to show unease like his men, so he ignored the urge to draw his sword. And that were his final mistake.”

He raked his finger across his throat, flung his hand wide, and teetered from the effort. The wild gesture brought even more eyes to him as he continued:

“Blood splashed across the faces of his men, and they turned to see a demon of a man materialize out of the shadows―a shadow himself. He flourished his sword and asked them who would be next. These were brave men, the whole lot, and they drew their swords, but it were for nothing. The Shadow cut them down one by one. A neck here, a heart there, until finally the last man was run through and pinned to the door while he tried to alert the men inside.”

Able frowned then let out the breath he had not realized he had been holding. Why he’d hoped a drunk sailor would tell a true account of the origins of a rebellion was beyond him. Still, he settled in to listen to the rest, since even if the old fellow noticed his disappointment in the sea of faces, it wasn’t discouraging him.

“But what were ten more men to the Shadow? He strode through them prison halls like he owned them, black cape billowing behind him, death in his wake. Once he was satisfied he would not be interrupted, he turned to the prisoners and said, ‘I am looking for men with hearts brave enough and backs strong enough to rise up and throw off the shackles of our Larbant masters. This is our land, not theirs!’

“And the Bor prisoners? They cheered and swore their strength to his cause! He freed the convicts and led them through the night until dawn. There, they were at the castle, where even more enforcers prepared a tax convoy for its journey south. From the black woods, the Shadow pointed to the heavy carriage covered in locks, and said,

“‘They’ve been taxing our people, taking food out of our children’s mouths, saying it belongs to the Larbant king now, that we are his subjects. Well, I say we are not his subjects, and he will not steal our property from us, so let’s take it back!’ And take it back they did, and they been taking it back ever since!” The storyteller bowed and came his closest to toppling off the table, all to a cacophony of whoops and applause.

“What were you saying about the decla―er, declation?” asked another, quieter voice, startling Able from his thoughts about the tale.

“…declation?” Able turned from the old man’s spectacle back to the young man across the table from him with a puzzled frown.

“You said ninety degrees, and then…attitude and then you were explaining declation?” The speaker was quite tall and already filled out with well-muscled arms, but his near-black skin was too smooth to have seen much of the elements. He was probably no more than eighteen, though Able wasn’t greatly his elder at twenty-five. His bright black eyes stared at Able with an intensity he was unused to.

There had been three other young deckhands sitting here when Able had taken it upon himself to fix the mess they were making of finding one’s latitude. Apparently, they had left while he was distracted by the theatrics, which was more usual and probably for the best as well; they would learn proper sailing techniques once they were ready to listen to their elders instead of each other. That Able had bothered to get up from his preferred corner by the bar and say something was probably a sign that he had a sheet loose himself.

“Ninety degrees, then subtract altitude, then add known declination,” Able corrected. “That is the formula for finding your latitude. Altitude is your target star’s distance from the horizon and declination is its distance from the equator. Those also need to be measured in degrees, of course, for the formula to work.”

“But,” the young man frowned, his brain working hard, “why degrees? Why ninety?”

“It’s just the standard used by the Royal Navy of Larbantry,” Able shrugged. “You could call it beans and use five of them, so long as you were measuring everything else in beans. The formula remains the same. But using the standard is simpler since they publish almanacs that list the declination of the sun and stars throughout the year.”

Editorial critique: One tiny detail before we begin. The apostrophe goes at the front, in “’twere”. If you remember the reason for an apostrophe in the first place, to denote a missing letter, “twere” is a contraction of “it were”, so the missing letter is the leading “i” of “it”. There are a number of such words, “’twas” and “’tis” are other examples, for which this holds true—worth remembering.
This is a good beginning, well-written, keeps up a fairly keen pace. Beyond the “’twere”, the technical standard is excellent, a few punctuation quibbles, and one sentence where the subject and object are ambiguous (Is it the young black sailor who is unused to staring with such intensity? Or is it Able who’s unused to being stared at?).
My concern here would be that you’ve disguised an info dump (the fact that these people are oppressed by the Larbant nation) in a “story within a story”. You’ve disguised it very well, but it’s still an info dump. I suspect we never hear from the old timer who’s telling the story again, yet you’re giving the opening few pages of your book to him. I suspect Able is the true main character, and he is hearing a retelling of something he knows very well, purely for our benefit. This structure is very similar to what is called “maid and butler” dialogue at the beginning of stage plays, where two minor characters come to the front of the stage and, while doing nothing very much else, discuss at length things that they both already know for the benefit of the audience. I suspect the few specific details contained within this colourful vignette could be more organically filtered in to the story in a myriad of other ways, but you saw this scene in your head and were struck by its vivid impact. I think, unfortunately, it has less of an impact than you think it does. It is entertaining, no doubt, but the real start of your story? I don’t think so.
You mentioned in your notes that you were concerned that this beginning might give a “bad impression”. I wonder if it isn’t this “story within a story” structure that’s giving you pause for thought.

One final thing. You’re very brave discussing celestial navigation in the very first few pages of your book. It might be better left until later, while we find out a bit more about Able, what his situation is, why that situation is about to change, and what he intends to do about it, which is probably what your story is actually about.