I use AmazonDCV for my clients to connect to Linux EC2 sessions. I would like add a watermark, and I can't find the documentation

Based on the information available, I don't see specific documentation about watermarking functionality in Amazon DCV in the provided sources. The sources cover general information about DCV sessions, troubleshooting, and integration with Amazon Linux 2023, but don't mention watermarking capabilities or configuration.

To determine if you're running a virtual session, you can use the dcv list-sessions command. Virtual sessions are only supported on Linux Amazon DCV servers and will be listed with "type:virtual" in the output, while console sessions will show "type:console".

For example, if you see output like:

Session: 'console' (owner:dcvuser type:console)

This indicates you're running a console session, not a virtual session.

Remember that you can only have one console session per server. To support multiple sessions, you need to disable console and switch to virtual sessions. Virtual sessions are created and managed by Amazon DCV users, and each virtual session starts its own X server instance.

Without specific information about watermarking in the provided sources, I cannot provide the exact configuration needed in dcv.conf or the dcv create-session command-line parameters for watermarking functionality.
Sources
Understanding Amazon DCV sessions - Amazon DCV
Could not create session. The maximum number of sessions has been reached | AWS re:Post
Troubleshooting Virtual Session Creation on Linux - Amazon DCV

While I am not aware of any watermarking features, you can check DCV release notes for any enhancements that may help.

If you want to customise the DCV user experience, i.e. to build customized web applications, take a look at DCV Web Client SDK