THere’s an option to use the Siri voice. However, there appears to be a bug, so that this only works when you have the default Siri voice enabled on the paired iPhone.
I use the South African male voice for Siri on the phone, and would love if my watch would use the same. Sadly, the bug prevents this.

Apple is aware of this issue, but has failed to correct it.

The only way to change the voiceover voice independent of changing the Siri voice is to switch the language of the Apple Watch.

To do this, open the Apple Watch app and go to general, language and region, switch the language to one of the English alternatives, make sure the custom option at the top of the page is selected and not the mirror option, and the voiceover voice on the watch will automatically change within about 30 seconds.

This narrowly expands your voiceover voice options but at least it does something. You still don’t get to pick a voiceover voice. English United States has one voice, English United Kingdom has one voice, English Australia has one voice, etc.

Additionally, you can expand your Siri voice options by doing the same thing in the iPhone settings. To do this, go into the settings for your iPhone and go into the Siri and search option below the accessibility option and change the language setting to an English alternative. Again, narrowly expands your options but at least it’s something.

I hope this helps.

I don’t think you can, I’ve looked because I hate the voice on mine

nope, we can't change the voices more than what you found.

I know you can change the voiceover voice on iPhone in the Voiceover settings menu, but I don’t know if this carries over to Apple Watch, I can certainly test this out for you though!