Wow, this is amazing in so many ways that I don't even know where to begin!
I guess the obvious place to start would be the character. I think you did a really great job capturing the design while also making adaptations to your own style. It has just the right balance of features that make it recognisable and features that make it look unique and different - after all there's no point you drawing it exactly the same as I would, because that defeats the purpose of getting another artist's interpretation! I especially like the way you drew the fins on the head and back, they just look so cool and fish-like. I also really like the pose and facial expression - combined with the lack of emphasis on the claws/teeth, they make it look so peaceful and graceful. It's nice to see that side of a creature that is perceived by the people in its universe as being vicious and bloodthirsty!
I also have to say you did a fantastic job with the background - way beyond what I was expecting from the price or even from the sample you posted in the commission journal. I love the details in the water and the way it subtly blends into the sand at the character's feet. The silhouettes of the plants in the foreground also add a lot to the feel of the image - the way they cast shadows on the character really make it seem like you're peeking through the bushes to get a secret look at it without it noticing. I think that works really well with what I said before about showing a normally-vicious creature as graceful - it's like you're getting a secret peek at what it's like when no one else is around. I can totally imagine this being like, some kind of award-winning nature photo featured in a wildlife journal - "Witness the secret life of dobhar-chus! Our photographers risked life and limb to capture these stunning images!". I mean . . . if photography existed in their universe.
OK I should probably stop rambling before this becomes an essay, but I just wanted to put as much as I could into words about how well this turned out. Thanks so much!