When 10.3 was released. I gave up my JB on my 6S+ and 5S to upgrade. I also upgraded one of my three watches to whatever the latest version was that came with 10.3. I quickly downgraded the phones back to 10.2.1 and that still left me with two phones jailbroken plus I bought an SE on 10.2.1. While I still within my exchange period at BestBuy, I exchanged the watch that I upgraded and got a new watch on 3.1. Since I have so many iPhones, three of them being used on T-Mobile, I was not willing to be have an Apple watch I could not use on any of my iPhones. BestBuy asked me why I wanted to exchange the watch and I told them. The lady had no problem with my reasons.
It may or may not be hard to find a watch on 3.1. The watch I exchanged was my Nike+ before the upgraded bands came out so all watches any place had in stock were still on 3.1. Like mentioned above, you'll probably have to buy used and ask every seller what version the watch is running.