Because the scripture called it a lake. Scholars go back to the original Greek in which it was written and study the words carefully. Your next questions will be about how those words are interpreted, whether the things said then still have relevance today, whether the words really were divinely inspired, and so on.
What are you really trying to say? Are you lost and wanting to know more about whether there is a heaven and hell? Do you know what will happen to you when you die? Are you sure?
I believe the Bible. I am curious about the reference to a Lake.
Everyone was well aware of the Oceans.