Unfortunately, the Nokia 8 doesn't level up with the other flagship on the market (e.g. LG G6, Galaxy S8, iPhone 7) that sports a IP68 protection rating. The IP68 ensures that the phone can survive when submerged underwater for 1.5m depth and up to 30 minutes. The Nokia 8 however is only certified with IP54 rating.
The x4 in the dual-digit rating number means that the phone is protected against Spraying water. Whether the 5x refers to the ingress of solid objects, which according to the IP rating table, this means that it's "Dust Protected". Just for comparison, the Samsung Galaxy S8 has a IP68 certification. The 6X means that it has the highest solid particle protection "Dust tight" and the IPX8 means that it can be immersed in depth of up to 1.5m for up to 30 minutes.
I'm sure there will be many people that will try out to see how durable the Nokia 8 is by putting it into various durability tests (e.g. heat, damage from falling, scratches, etc.) and I'm sure one of them will be testing if it can survive underwater for X amount of time. You might want to search YouTube for some Nokia 8 durability tests to see those videos, which I'm sure will appear not far after the phone is released.