Hi there, Alishap86!
Thanks for reaching out to the Apple Support Communities! That's a great question you have about using the Apple Watch without having to buy another cellular line for the Watch. Check out this article to Set up an Apple Watch for a family member:
* A cellular plan isn't required to set up an Apple Watch for a family member, but is necessary for some features."
Per the article, a cellular plan is not required for setup, just for certain features. You'll find more information about the features you can use without Wi-Fi or an iPhone nearby in this article to Set up cellular on your Apple Watch.
Thanks again for the question and take care!