Hi Ryos... This was an interesting and enjoyable submission. I wouldn't say that the story was incredibly exciting or dynamic for me, but it had some really good things going for it.
What I liked most about it was the very strong sense of intrigue we get from your setting. I love how the whole society is basically trapped in a tiny strip of land, bound in by both the ocean and desert. And kudos to you for building a society which adheres to those limitations very believably. (ie, they worship an ocean god, for example). Quite honestly, this setup is one of the most interesting I've read in this group so far. Well done!
W/R/T your setting. one thing I wasn't certain of was the level of technology available. At times it gave me the sense that it was in a medieval period (a dungeon, priests, ships, etc), but at others I got the sense that it could be modern day (the docks, levels, catamaran, etc). I am pretty certain you intended this to be a high fantasy story ,but just keep in mind that if you picture a modern-day world for this story, it still holds water.
"Selendy, scheduled to depart on her final voyage with the first light of tomorrow's dawn, lies sleeping away her last hours of life."
It wasn't quite clear to me that the priestess was quoting something from a paper. And if she was, the second half of the sentence ("...lies sleeping away...") seems like an odd thing to describe on some document.
"30 miles, child. That's not a swim you just make. Only the wrongfully convicted are blessed with the strength to escape their execution."
I was confused by this. Was the priestess telling her how far it was to escape her prison? (Like on Alcatraz or something?) It wasn't until I read a comment in this thread (Thanks, Reaves!) that it dawned on me that the prisoners are dumped 30 miles out and told they can live if they make it back. Maybe I'm just thick.
But if not, then consider clarifying this early on. It also would help clarify the priestess's motivations a little.
Oh, and, because others here have discussed it... I picked up on the "Right / Left" thing pretty quickly.
Some of the flashback transitions felt a little awkward to me. The first one... where she remembered killing the Dockmaster... was okay. But the next one where she just rolled over and then dreams every detail of her backstory felt a bit of a stretch.
With that, she dove backwards off the stern of the catamaran and swam gracefully downriver.
I admit I chuckled a little when I read this. It was a funny image of an old lady jumping ship and swimming. Was this event intended to be supernatural? Or is the old lady really just a good swimmer (due to the fact that she's an ocean god priestess?)
Also, just a nitpick, but wouldn't she be swimming UP-river if she was headed back?
I want to know more about the old lady's motivations. (Which is a good thing. I want to read part 2 now).
Finally, I think you might benefit from doing a pass over this to look for instances of passive voice. I'm no expert in finding that (as you may know well from my own submissions!) but there were some instances where the sentences could have been more direct.
Looking forward to part 2. Well done.
J