On the one hand, the extreme heat causes the metal parts to expand faster than the rubber gaskets can accommodate and this can create minute openings which allow steam into the watch. Additionally the heated environment warms the air that is trapped inside the watch case, a closed off environment, and then the heated air has no way to escape which causes condensation inside the watch.

Remember it is NOT actually the water that pressure-seals a watch case but the depth traveled under the water which applies atmospheric pressure externally on the watch case and thereby achieves water tightness.

As such in the shower / sauna / hot-tub you don’t have external (depth) pressures exerted on the watch case and therefore it is a Swiss “rule of thumb” to not expose a watch, even a diver’s-rated watch, to any of these steam-generating conditions. That’s not to say you can’t, it’s just not recommended. Additionally harsh solvents, such as shampoo and soaps, can affect the o-ring gasket rubber which can cause degradation and/or failure, allowing water to enter the watch case and ruin the expensive internal components (dial, hands, movement, etc.).

POSSIBLE REMEDY FOR STEAM:
If you do have steam or fog in your watch, we suggest placing your watch with an open crown inside a bowl of rice, which allows the water to slowly be evaporated and sucked up by the rice. We know this sounds strange, but it can work; however we recommend not showering with it. Then send your timepiece in to be serviced and get the gaskets replaced.