Find the geonetwork database. This is a file called gn.h2.db but where it lives is dependent on what webserver you are using.
On windows the path should not have colons in it- eg C:\path\to\database will look like C/path/to/database
Download platform independent zip file from http://www.h2database.com/html/download.html and unblock/extract it
At a command prompt cd to the h2\bin directory
DO NOT USE THE bat OR sh SCRIPTS TO OPEN THE WEB CONSOLE AS IT SEEMS TO CORRUPT THE DATABASE!
Enter the following command:
java -cp h2*.jar org.h2.tools.Shell
When asked for the URL this will be of the form: jdbc:h2:path/to/gn (not gn.h2 or gn.h2.db)
The generic username is www-data as is the generic password
Note that you may need to shut down geonetwork, and/or delete the additional file gn.lock.db, and/or give write permissions on gn.hd.db
When you successfully connect you will get a prompt like: sql>
See http://www.h2database.com/html/commands.html for command reference.
To view a particular table such as the schemas:
SELECT * FROM SCHEMATRON;