The difficulty comes because jailbreaking requires an older iOS version, and WatchOS will refuse to pair with an iOS version older than itself.
This is, by and large, an artificial requirement. Managed incompetence from Apple goons not bothering to code in any sort of backwards compatibility because it helps their business to encourage upgrades. A bit like how updating iOS meant you had to update iTunes also, as if the syncing methods were changed significantly between versions, but I digress...
This usually means having the latest version of iOS or something very close to it when you buy the watch, but if you can find one with an older version installed, or jailbreak the latest version of iOS (can now be done tethered on iPhone X or lower), then it will pair.
There is some wiggle room, depending on how long it has been on the shelves for, especially if you buy used. And you might get an earlier point release if you buy a new watch late in its refresh cycle. If memory serves, mine came with 5.1.1 out of the box, at a time when 5.3 was the latest version. It's not getting WatchOS 6 though, unless I bite the bullet and update iOS on my phone.
Once it is paired, you don't need(?) to worry about it updating automatically to an incompatible version of WatchOS, otherwise iPhone 6 users would be SOL from this. I still put the TVOS beta profile on my watch, just to be on the safe side, but I'm not sure it's really needed.