How reliable are waterproof phones?

I want to have a waterproof phone that I can realiably wear when I am windsurfing. During a session it might get wet and might sink for a few centimeters for a couple of seconds from time to time. I bought a **refurbished Huawei P20 Pro** for the purpose, **IP67 waterproof**. I had watched videos on Youtube where it would be under 1.5 meters water for 15 minutes and still be working fine. After a short session on the water with this phone, where it got wet but never sunk, the next day the phone was **dead forever with water damage**. I imagined that, being refurbished, it might have been hacked by the previous owner. I got my money back and bought the same phone again, also refurbished. This one was really new, in the box with all the gadgets. I took it on a session, this time in a cheap "waterproof" pocket. At the end of the session, there was some moist in the pocket. Next day the phone was dead forever. Again. Was this hacked too? I don't think so. I got my money back and bought a **refurbished Samsung S9**, which is **waterproof IP68**. Now. I am not going to bring it with me on the water until I am sure it will survive. I could buy a IP68 protection for it but wouldn't it be overkill and actually nonsense (IP68 + IP68 = IP68) and maybe still let water in? Are these videos on Youtube realistic? Maybe 10 minutes after the video the phone becomes useless. **Are the IP67 and IP68 certifications reliable** for general public phones? I am sure they are when it concerns professional equipment. Thanks for your thoughts