"The fish swims in the sea"
Translation:Fisken simmar i havet
It's not quite that easy. "Sjö" in Swedish used to be able to mean sea in the oceanic sense - hence names like Östersjön, which clearly isn't a lake. Over time, we've gradually lost that meaning. It remains in names, but it's not really used as such otherwise - at least not outside of expert terminology in e.g. academia and industry contexts.
So you're not wrong per se, but "sjö" in this context has an archaic sense which shouldn't be taught to beginners. "Sea" cannot mean "sjö" as in English "lake".