Devils advocate, most off the shelf mass market electronics are actually quite reliable. Having custom made hardware often means poor firmware support, extreme costs, and difficult to debug.

Nothing wrong with using off the shelf electronics, especially since the interior of the submarine is atmospheric pressure.

I have no idea if they actually had spares, but there's something to be said for having three $30 off the shelf parts over one $200 custom part, provided that failure isn't immediately catastrophic.