Wrong again. Read the IP ratings.
https://en.wikipedia.org/wiki/IP_Code
IP68
6 - No ingress of dust; complete protection against contact (dust tight). A vacuum must be applied. Test duration of up to 8 hours based on air flow.
8 - 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.
Samsung gave it a IP68 rating because they tested it in one meter of water for 30 minutes. Samsung does say to not use it while under a water fall, or other place where you'll encounter jets or streams of water. That is because it is not IP69K rated. I live in Hawaii and I have taken my S8+ into the ocean WITHOUT a LifeProof case with no problems. What did not do was take it below 1.5 meters. When I got the moisture detected notification, I just let it air dry for a couple of hours and it worked just fine.
Here are the specs for LifeProof.
"IP-68 INGRESS PROTECTION
The international standard for Ingress Protection (IP). 6 and 8 are the highest ratings for solids and liquids. IP-68 rated cases withstand circulating talc for 8 hours and water immersion to 6.6 ft / 2m for 1 hr."
Why LifeProof | LifeProof
P.S. If you do take it into salt water, rinse it off in fresh water as soon as you can. I do that with my GoPro 5 and the case for my GoPro 4.