I remember how sad it was when John Denver died in an experimental plane he was piloting. Regulations for experimental aircraft were changed following that crash due in part to public attention.
I imagine there will now be changes to regulatory oversight of experimental submersibles.

I flew in quite a few experimental aircraft with no concerns what so ever.
But I also knew that in my time they were built to specification, by people that were
experts and their work was inspected for compliance. All our flights were planned
in detail.
Safety is #1, you fly unsafely and you may die in an experimental aircraft.
I spent twenty years in Boeing Experimental flight test.
Never lost a plane, we broke a few, but never crashed one like Airbus did.
Most private built experimental aircraft that fail are due to someone hot dogging
it and pushing it beyond it's or their limits.
Most experimental aircraft are over built for safety, and their flying limits are set well within
design limits. eg the wing can safely handle 10g's, but the flight limits will be set to 8g's max.
Sub was poorly designed , poorly built, and exceed material design limits.
how was it tested? They found out the hard way. it wasn't.