Hi
Whether or not to fit a screen protector is a matter of personal preference.
Ion-X glass is less scratch resistant than sapphire crystal. Scratches aside, a screen protector may also provide some measure of protection against impact-related damage.
You may wish to consider, for example, products from:
https://www.armorsuit.com/pages/apple-watch
If you have not yet done so, then you may wish to consider purchasing AppleCare+, which provides for up to two instances of accidental damage (but not cosmetic scratches or damage), each subject to payment of a service fee / excess.
If AppleCare+ is available to purchase for your Apple Watch, then it may be offered as an option when checking your service coverage here:
Check Your Service and Support Coverage - Apple Support
Otherwise, to learn about any AppleCare+ policy or plan that may be available to buy in your country and the applicable time limit for purchasing it, follow this link, select your country and, on the next page, see the information within any section headed "Can I buy additional coverage for my Apple Watch?":
Apple - Support
Details for the US and UK, for example - including instructions at the bottom for buying AppleCare+ - are detailed here: