The iPhone X is not waterproof. Generally iPhones are built to withstand some water penetration but won’t always survive a drop in deep water or or being submerged for an extended period of time. Read...some intense rain might be okay...the toilet, you might not be as lucky. Don’t go swimming with your iPhone.
The iPhone X does have a level of dust and water resistance. Apple has noted that this resistance is not permanent and may decrease with time. The water resistance might improve slightly with a case but the iPhone X is not technically waterproof. You should always let your iPhone dry out completely before inserting any cables or trying to turn it back on.