Besides the top fin, which looks a little weird to me, it’s nice, I would start texturing and adjusting the lighting (or making more models, don’t know what you have in mind for your finished scene ;)). For a ‘serious’ render it still needs a lot of work of course. Also, with these sharp edges (and no texture) it kind of looks like it’s been folded out of paper. Don’t know if that’s intended or not, could make for an interesting style
If you don’t want it look like that, try activating smooth shading in the tools panel. Or, depending on how clean your topology is and how low poly you want your scene to be, try adding a subsurf modifier.
But personally I would leave it like that, I don’t have much experience (frankly I haven’t got any) with low poly scenes though.
I am aiming low poly. So, I won’t be adding much texturing. I’ll, likely, end up changing the materials around a bit to have it look less paperlike and more fishlike. But, it is not intended to be life like, that’s for sure.
I agree with you about the dorsal fin. I’m still trying to find a geometry I like with that one.