There are companies dedicated to certifying maritime vehicles and standards against which they need to be built and tested, and Stockton Rush explicitly rejected them because he felt that they were stifling to innovation. Additionally, the people who told him that he was needlessly endangering people's lives were experts in their fields. And again, it wasn't just "someone", it was virtually everyone with experience in super-deep submersible operation.
The idea that just because there are often naysayers who claim some big projects are impossible doesn't mean that there aren't situations where they should absolutely be listened to and taking passengers on a submersible down to the Titanic is absolutely one of them. This guy played fast and loose with safety and if it had just been him down there, that would be fine, but he managed to bamboozle a bunch of innocent people into it as well.