This article is oldish, it was written all the way back in 2004. I just wanted to share this because it shows another facet of the submersibles world, where every day people build their own submersibles from kit plans. I remember kit plans from back in the day before the internet, when passionate people would subscribe to magazines like Popular Mechanics and (usually) use tons of fiberglass and other materials to build whatever functioning contraption their hearts desired. This is on other end of the spectrum to billionaires descending thousands of meters below the ocean surface, but as of the date this article was written (2004), there were no fatality accidents involving certified submersibles. (I don't know if that safety statistic has changed since 2004, but I was quite impressed with that record either way.)
Here also is a link to one of the main personal submersibles communities, called http://psubs.org/ Personal Subs dot Org.
There are many other resources out there. I've been spending a little time celebrating them and learning something about this fascinating hobby and community.
Cheers.
edit: Here is a list of known incidents involving submersibles since 2000. With the exception of Titan and Nautilus (The Danish submersible intentionally sunk as part of a murder plot, in 2017) , all have involved 1 or more submersible (most often appear to be submarines) involving naval vessels.
https://en.wikipedia.org/wiki/List_of_submarine_and_submersible_incidents_since_2000