Ultra 3 with Cellular

[IGWT15]

I need to confirm if it's a MUST that I have iMessage enabled in order to be able to receive and sent text messages when my iPhone is Off or out of battery? I am able to make and receive calls when my iPhone is off or dead.

 

[doogm]

See https://support.apple.com/en-us/108300, particularly footnote 2.

2. To receive SMS, MMS, or push notifications from third-party apps on your cellular Apple Watch, your paired iPhone must be powered on and connected to Wi-Fi or cellular, but it doesn't need to be nearby. You also need to be signed in to iMessage on your iPhone to use Messages on your watch.

 

[Apple_Robert]

This answers your question.

Use your Apple Watch without your iPhone nearby - Apple Support

Learn what you can do with your Apple Watch when your iPhone isn't with you.

support.apple.com