Apple Watch

Read the following

https://support.apple.com/en-us/HT205547

pay attention to all the footnotes

The GPS only version can do "Everything" the Cellular model can do when your iPhone is nearby. The Cellular version can continue to do things, when the iPhone is left behind while you go for a run, hike, bike ride, swim, etc....

That is correct. Bob is correct in making sure you read the footnotes. There is one especially when it comes to certain message types, such as SMS and 3rd party notifications that requires the iPhone to be powered on and connected to the network for those notifications to work, even though it is not necessary for it to be nearby.

Yes, as long as the iPhone is nearby and connected to your Watch, you can text.