This might be a case of under-promise, over-deliver.
IP68 means absolutely no dust whatsoever and:
"The equipment is suitable for continuous immersion in water under conditions which shall be specified by the manufacturer. However, with certain types of equipment, it can mean that water can enter but only in such a manner that it produces no harmful effects. The test depth and/or duration is expected to be greater than the requirements for IPx7, and other environmental effects may be added, such as temperature cycling before immersion."
In the S8's case, the manufacturer specified conditions were "submersion in up to 1.5 meters of fresh water for up to 30 minutes", from http://www.samsung.com/global/galaxy/galaxy-s8/performance/.