I had the Series 4 awhile back, and it died after a very shallow dip in the water. Took the opportunity to upgrade to the Ultra 1, and I see absolutely no reason to upgrade past it. It isn’t fundamentally different than… well, the other Apple Watches, beyond having a button you can’t set to do anything particularly useful and having an ever so slightly larger screen (which is the main reason I got it).
Well, to be fair, my old Series 4 was a refurb, so the waterproofing likely wasn’t factory fresh.