There are waterproof cases you can purchase specifically for using an iPhone underwater. A company called watershot (https://www.watershot.com/) makes them for a variety of smart phones and tablets. I don’t know what sort of guarantees they include with their products against water intrusion, and I have no personal experience with their products. There are other companies that make similar cases.
As to what people post online, well, people do all sorts of ridiculous things with expensive electronics and put pictures, videos and posts online. Many of them who’ve posted here clearly have come to regret those activities.