‘Hey Siri’ is purely optional in watchOS 5
 

Why do you need to say Siri when you've already got your Apple Watch hovering right in front of your face? Soon, you won't have to . WatchOS 5 will drop the need for you to vocally summon Siri, instead, as soon as your raise your device up, it'll sta...

https://www.engadget.com/2018/06/04/...s-5-wwdc-2018/