Its really not consider as water proof only water resistant. Iphone X would be able to resist the penetration of water to some degree but not entirely. This is mean that this device won't be imperviosu to water.
Up to date their isn't really a device that's under Apple that is considered to be water proof. All are to be considered as water resistant. If you truly desire the Iphone X and would wish to use it under water the only way to do so would be with water proof casing.
I learned this the hard way when we went to the beach and thinking that my device is considered to be waterproof I used it underwater and It damage some functions on my phone.
Hope this helps you in some ways.
As i heard, it is but only up to a certain point, basically like all the other waterproof phones. Don't exactly remember which dept it can go up to, forgive me for that. Anyways, if i were you, i still wouldn't test it, you never know if there is a fabric mistake or not..