As a child, I had a $20 plastic submarine that was powered by AA batteries. It could sit at the bottom of a pool for days on end, without water ever getting int

As a child, I had a $20 plastic submarine that was powered by AA batteries. It could sit at the bottom of a pool for days on end, without water ever getting into the battery compartment of the engine module (which was sealed with a rubber o-ring). I bet there are hundreds of counter-examples to your claim that water-proofing with an o-ring is an engineering challenge of the utmost difficulty.