Would replacing my screen jeopardise the watch’s waterproof properties

"First it is water resistant not waterproof

https://support.apple.com/en-us/HT205000

If you do the repair, any remaining warranty you have will be kaput.

Replacing it yourself and getting as good a seal as the factory did is unlikely.

The part is out of stock and expensive. I'd call Apple as see what they are charging first."

"There's no way to guarantee that it would still have the same water resistant rating. Also, the replacement adhesive strips I have found so far are a bit thicker than the OEM one. So after you re-set the screen in place, it sticks up a teeny, tiny bit higher than it did originally. Literally, we're talking a fraction of a millimeter, but it is enough to feel. And if you really look closely, you can see the difference too. Does that matter? Maybe, maybe not, but personally, I worry a little bit about the edges of the screens being that much easier to chip.

Personally, before I did anything else, I would try picking up a good screen protector (probably one of the plastic ones like the stuff from Armorsuit, not a tempered glass one) and see if that covers up the scratch enough to not bother you so much."