is that the sea or a lake should be accepted
Or even better, "is that the sea or a lake"
Yup, and actually probably more likely to be heard in real life...
Agree with other commenters saying "the sea" is a better English translation in this case. Maybe because Britain's an island, we don't have "a sea" in everyday usage, usually "the sea".
It looks like this question is about The (?) Caspian Sea