perhaps some one with scientific background could comment on this. Is there a difference between pressure applied to the exterior and seals of a housing by air or water? My uneducated guess is that there is no difference. However, given water is much denser than air would this density difference be an influential factor?
The housing has no way of knowing if the pressure inside is 0 psi, vs 15 psi outside, or 300 psi inside vs 315 psi outside. It only responds to the pressure differential between the inside and outside.
However, water is more viscous than air, and it has a higher surface tension, so I would expect a given size of leak to leak much faster for air than for water. So, if anything, the vacuum systems are conservative.
The main difference is of course that the biggest pressure differential you can produce with a vacuum is atmospheric pressure, about 14 psi. This is equivalent to roughly 33 feet of depth in salt water, and that is only if you manage to pull a near perfect vacuum. So if you go to 60 feet, then the housing is seeing a bigger pressure differential than you tested it to with the vacuum. I guess the housing or the o rings could deform a bit more and so something that doesn't leak at 30' might leak at 60'. I'm not saying this will happen, but I can see how it might be possible.
Still, my understanding is that most floods are because of damaged or non-existant o-rings, or because there is crud on the o-ring. I suspect that while a vacuum system might not prevent 100% of floods, it would probably prevent the majority of them.