Concept: it's a bit strange setting for the theme given, feels like the fish idea came first and the necromancy stuff was just an afterthought.
Gameplay: it's simple and nothing new in design, but it works pretty well, found no bugs, ran smooth. The upgrade system was a nice addition also. The bossfight is pretty cool, I like the varied moveset of the boss. It felt like it should've given a bit more telegraph of its attacks though, I many times felt like I had didn't have any time to react.
Art: it's fish sprites and background is nice and the skeleton is amazing with really good animations. They don't really fit together well though, the skeleton feels like it's from another game. Also are the fish and the background from an asset pack? I don't see it declared anywhere but it's a very different artstyle from the boss so I assume it's premade. Please correct me if I'm wrong though.
Audio: music is good and fitting, sound effects are nice as well.
Overall it's a decent game though feels very random at times.