The Ion-X Glass is essentially the same display as the iPhone. It's fairly scratch resistant, but not nearly as the sapphire display. Sapphire is the second hardest material to diamond. It's very To difficult to scratch the Sapphire display on the stainless model.
That said, the Ion-X Glass from my experiences is fairly resilient, it uses glass crystal, but when I did own mine, it scratched and I upgraded to the Stainless model with the sapphire display. I still have zero scratches on my stainless model Watch.
Here is a similar thread where a forum member scratched their Sport model and out a screen protector on it. Which I would highly recommend.
https://forums.macrumors.com/threads/got-my-first-scratch.2040872/
This would be the screen protector I would recommend if you decide on one.
http://target.georiot.com/Proxy.ashx?TSID=3088&GR_URL=https://www.amazon.com/gp/product/B00UH3K244/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1[/QUOTE
Is that a wet application screen protector?