To Celluar or not?

I had an old Apple Watch that did not have cellular. I could pair it with my phone to answer calls and use certain apps.

My question is, with the newer models of Apple Watch. Do I need the Celluar option on the watch? In particular, Ultra 2 only has a cellular version. Can I pair the Ultra 2 to my iPhone 15 without activating the cellular service? What features will I be missing if I do not obtain a cellular plan for my Ultra 2 watch? I will leave my phone on the boat and dive with my watch. Will my dive still be logged?

Tom