Built like an airplane out of materials that were past their shelf life even for airplanes:

Only one thing concerned me: He said he had gotten the carbon fiber used to make the Titan at a big discount from Boeing because it was past its shelf-life for use in airplanes.

I asked him if that weren't a problem. He replied that those dates were set far before they had to be, and that Boeing and even NASA had participated in the design and testing of the Titan.

It is a conversation I have thought about a great deal over the past week.

https://www.travelweekly.com/North-America-Travel/Mission-Titanic-Part-2

Also, Boeing denied being involved, and NASA stated that they had given advice but not been involved in testing:

https://www.usatoday.com/story/news/nation/2023/06/23/titanic-submarine-submersible-implosion-recovery-live-updates/70349511007/