i got my iPhone X water damaged and my varanty doesnt cover it. If i pay apple care now, can they repair it?

You are correct, the warranty does not cover water damage. You can make a Genius Bar appointment and see about an out of warranty replacement. See this for costs. iPhone Repair - Official Apple Support

If you do not have Apple Care at this point, you cannot purchase it after the fact.

Apple Care+ can only be purchased while the iPhone is functional, and is certified by Apple to be so.

You cannot buy Apple Care after damage has occurred.

All you can do at this point, is pay the out of warranty replacement price.

iPhone Service Pricing - Apple Support