I've read the first two chapters and it seems like an interesting concept, has a nice mundane magical feel with enough hints of a bigger world that it feels real. I have a good idea of what Irian wants, which is always important for a protagonist, and I'm intrigued by the gills. Doing phonetic accents is always a tricky thing but I think you're pulling it off pretty well.

On the other hand, I would say that the scenes feel kind of, and this isn't meant as a pun, shallow. I don't know if I can explain this very well, but each scene feels like it's about just one thing--Irian scaling the merboy, reading the letter, and so on, when you could be skimming over some of the transition bits and building more into each important moment. Like the scaling scene, you've got the basic idea of establishing how this job works, mostly through narration/general description, but you could have some development of the relationship between Irian and Tarn--are they friends? Rivals? Do they mostly ignore each other? Is Irian bored by his job or still trying to prove himself, or does he find it weirdly satisfying?

Basically I think you've got the concept of a fun fantasy world and an engaging main character set up, the story just needs to be more streamlined.