The back of my apple watch cracked

If it was damage caused by you, no, it's not covered. You will have to pay for a replacement. Unless you have Apple Care+, then those conditions apply.

If the damage was caused by a Manufacturing defect, then Apple will replace it free of charge.

For more info, see this page: https://support.apple.com/watch/repair/service

If you bought AppleCare + and it is still in effect it will be covered, other than that you will need to take it to your local Apple Authorized Service Provider to be assessed to see if it's a manufacturers defect. Cases made of aluminum or stainless steel rarely crack spontaneously.