Please, these things are not mechanical watches, which have been able accompany divers for literally centuries.

Speakers are trivially sealed, as are buttons. They are just a fancy electromagnet, or piezoelectric crystal.

Also both can work through induction potentially keeping them outside a "pressure chamber" and forgoing sealing altogether. Although I just want the steam of my shower not to mess up with my phone, not dive 100 m with it.

Not to mention headphone ports, usb ports, and charging ports. Also most phones have a vibrate function which uses an electric motor.

Wait, ports are considered moving parts?

Yes. Connectors like USB and headphones consist of metal "fingers" in a plastic insulating shell, perhaps with an outer metal shell. The contacts are sprung and need to move. So the simplest, most natural implementation has gaps around the contacts. There's also usually a thin gap between the connector on the PCB and the outer casing of the phone. If you want it sealed you have to add gaskets everywhere during assembly.

Not to mention sockets for SIM and SD cards. If you can take the back off the whole phone to change the battery, that makes it a lot harder.

Quite a few phones are waterproof or splashproof : http://www.techradar.com/news/best-waterproof-phone

Exact meaning of ingress protection "IP" ratings: http://www.dsmt.com/resources/ip-rating-chart/

Or if you want something cheaper: https://www.gearbest.com/cell-phones/pp_602673.html

Many ports, and notably headphone jacks, have springy pieces of metal inside to clamp onto the plug. This is what causes scratchy audio when you wiggle the plug in old devices, the clamping force has weakened.

Don't things move inside them? They are pretty hard to make water tight.