Is iphone XS max fully waterproof?
if any damage happen do you pay for apple care?
No. See the footnotes here:
iPhone XS and iPhone XS Max are splash, water, and dust resistant and were tested under controlled laboratory conditions with a rating of IP68 under IEC standard 60529 (maximum depth of 2 meters up to 30 minutes). Splash, water, and dust resistance are not permanent conditions and resistance might decrease as a result of normal wear.