I'd sooner use "bird" - the feeling of freedom is greater in the sky than in the sea. We usually say "free as a bird" for freedom.
How about, "carefree as a fish in the sea (could be)"? I suppose it depends where this fish is.
I checked, it seems "carefree as a child" is used often, but more as a comparison, less as an idiom. But I like the poetry in your sentence. :)