To be honest, a watch would probably be safe to shower with even without any gaskets at all...the tolerances of the screw down crown and case back are probably enough to withstand splashing. Immersion to depth for extended periods of time is probably more of a concern, and even the deep end of a swimming pool doubles the pressure from sea level and can cause a watch to fail if there's any weak links in the water resistance "chain."
That said, I'm of the opinion that most reps of the quality discussed on these forums would handle day to day use and abuse, not including water sports of any kind (swimming, diving, etc).
As assembled from the factory, the tolerances are close enough, and the gaskets provided are of enough quality, to make most reps water resistant out of the box, probably even to recreational scuba diving limits. I've said this before, but a watch is just a fancy "box" that protects a watch movement. The only places where water can intrude is the crystal interface with the watch case, the case back, and the crown (assuming no complications like chronograph pushers, etc)...it's not going to permeate the steel itself.
So as long as the crystal isn't defective, or has gaps between it and the case, generally a gradual application of pressure would only actually it MORE watertight, until the point the crystal fails catastrophically. The pressure actually seals it to the watch body as pressure increases.
The case back and crown are tight enough (if screwed down) that the pressure is highly unlikely to defeat them.
In the real world, however, the reason gen watches fail water resistance at the crown or case back is usually due to damage, wear, or improper installation. Damage can be from dust, hair, sand, or dirt, or exposure to a harsh chemical. Salt water dries and leaves a salty, abrasive residue, and even the simple act of unscrewing the crown to set the time can cause a lot of damage to the crown/tube interface. A case back can pick up dust or dirt or hair every time the back is removed, which is why most places won't guarantee water resistance after a battery change unless it's tested again.
The reason that rep watches fail is all of the above, plus the chance that the crystal, back, or crown is installed improperly or missing pieces, especially the gaskets or whatever glue they might use for the crystal. I've had a few come without case back gaskets...one of the things we pay for with a gen is extremely high quality control. One of the reasons reps are so cheap is because their QC is probably just a guy giving a hundred watches an hour a quick once over, lol.
I'm willing to bet that most of the aftermarket "waterproofing" we pay for is just a pressure test to ensure there's nothing from the above going on, and/or the correction of whatever errors they find (like a new or undamaged case back gasket, for example).
