Years ago, I used to use Data Architect. I don't know if it's still out there.
You could reverse engineer an existing schema into a relational table diagram.
Or you could go even further, and reverse engineer an Entity-Relationship model with an accompanying diagram. ER diagrams were really useful to me when discussing the data with people who were neither programmers nor database experts.
Sometimes a few manual fixups to the ER model and ER diagram were necessary before it was a useful communication tool with stakeholders.