Yes.
iMessage just needs access to the Internet, it does not care if that is via WiFi or Cellular, as long as it looks like a TCP/IP connection and that is all data connections care about.
Of course when you are not connected to a WiFi network, then you will not be able to send via iMessage, or use any app that requires data, such as current weather information, Find-My-Friends, etc...
Whenever possible the Apple Watch will use your iPhone's radios if it can talk to the iPhone via Low Energy Bluetooth (conserves the Apple Watch small battery).
This webpage can be useful in understanding the iPhone/Apple Watch interactions
https://support.apple.com/en-us/HT205547
Pay attention to the footnotes.
And when the page says something that implies the Apple Watch will use the iPhone's cellular data, you have to assume that will not work for you when you are not connected to WiFi.
Also if your carrier does not support a data connection, it will not support a Cellular Apple Watch, as the Cellular Apple Watch ONLY uses cellular data and depends on being able to do WiFi calling.
The GPS Apple Watch will use the iPhone for phone calls, especially if WiFi calling is not supported.