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. :)

I dont think so ) neither it cares, however ^__^

Ah, I didn't think of that meaning Meow. How about "worry-free"? Do fish worry much?

yeah, bird is better. but I would object to "carefree" - it causes unhealthy association with women's hygiene products let it be "free" or "light-hearted"