Maybe we must understand one thing.
How many times we bought new car, that is just not working properly, engine trouble, small mechanical errors, sometimes, some cars are just sloppy built or not up to the standards.
Now cars are built in 100 of thousands and errors designer or manufacture ones are being discovered during production.
Submarines are built at the best in their doesents, 10, 20, 30 in series, and there is no chance to discover small failiures until something bad happens.
Now small electrical fiers can happen on cars while you are driving, you know you feel smell, and see smoke coming out of the dashboard...
Now imagine that while you are 300 meters down the surface, and your engine just stopped for some reason...