No reason why an electrical socket can't be made waterproof and still allow conductivity (just need to make it capable of being a tight enough fit that the water bridging conductors is negligible, or take advantage of low-wattage over long distances like with waterproof headphone jacks). I don't think the 3.5mm headphone jack is actually a problem there, though. It can be tightly sealed from intrusion, and given the current and distance between contacts short-circuiting due to water is minimal. See
http://www.amazon.com/Aduro-Waterpr...883&sr=8-8&keywords=waterproof+headphone+jack as a cheap example of a product with a waterproof headphone jack.
The harder part of "waterproof" is that generally speaking the public doesn't understand what that means at all. There are standards for depth of water (and salinity of water) to which the product can be safely submerged, but people hear "waterproof" and assume they can take it to the bottom of the ocean and all will be well (see reviews on the above link for examples). The deeper you allow the device to go the harder it gets to maintain water tightness, as pressure increases linearly with depth but expense (meaning, monetary expense as well as design tolerances and necessity of effects on external product design) to achieve water-tightness is super-linear with pressure (in the shallow ranges; once you get to a certain pressure level the curve almost flattens out again). And, of course, having a bog-standard headphone jack indicates that any old headphones (most of which are NOT water-proof at all) can be plugged in, and people will blame the phone for any issues arising from their headphones shorting out while in the pool.
All that said, I'd love to see a toilet-proof (and, er, subsequent sanitization-proof) iPhone. I'd also love to have an iPhone which could take me jumping into the pool in an emergency if necessary (which caused the premature death of my old Treo 700 many a year ago). Hell, not having to worry about it dying in a sudden rainstorm would be just great too (appears the 6S is at this level, but not guaranteed as such).
