Why showering is hard on watches
A water-resistance rating tests the watch under steady, room-temperature pressure. Showering combines three factors that lab tests don't simulate: hot water (38-45°C) expands the rubber gaskets and metal case at different rates, opening micro-gaps; soap and shampoo are chemical solvents that attack the gasket compounds over time; and the rapid temperature shift between hot shower and cool ambient air cycles the seals, which is one of the fastest ways to age them.
The 'I shower with my Submariner every day' counterpoint
Plenty of Submariner owners shower with their watches and report no issues. The watch doesn't immediately fail. What does happen is faster gasket aging: the recommended service interval becomes 5 years instead of 10, and the gaskets start failing at 3-5 years instead of 7-10. If you're committed to brand service every 5 years, showering with a 100m+ watch is mostly fine. If you want to extend service intervals, take the watch off.
What's actually safe
Hand-washing is fine on any watch rated 30m+. Pool swimming is fine on 50m+. Snorkelling and beach use on 100m+. Recreational scuba on 200m+. Showering, hot tubs, and sauna are NOT safe on any watch you care about, regardless of rating. The temperature + chemical combination is harder on gaskets than the actual depth performance the rating describes. See: water resistance guide.