For v1.4 there was a PDF of the database tables and stored procedures that you could download (Visio-nopCommerce14DB). I can't find this for v1.5 on the site, is there one available or is it in the documentation now ?
Starprint has a great product "SqlServerprint".
Not free but worth the money.
You can use SQL Server Management Studio to create a database diagram. Select reuqired DB -> select Database Diagrams -> Create new diagram -> Select All tables.