Important rules

Panerai

Always ensure the crown is pushed right down and/or screwed tight before entering the water.
Never use a chronograph function while in the water (unless the watch is designed for that purpose) as water could get into the movement.
A watch professional with the appropriate equipment can test a watch's water-resistance.

Water-resistant: accidental splashing, slight humidity
Water-resistant to 10 metres: washing hands
Water-resistant to 30 metres: swimming
Water-resistant to 50 metres: showering, jumping in the water from a river bank, lake or poolside
Water-resistant to 100 metres: jumping from a diving board, jet ski, snorkelling
Water-resistant to 500 metres and more: any water activity

A hot shower exerts pressure of at least 5 bars (causing the gaskets to dilate because of the heat and pressure of the water)
Diving from 10 metres exerts 10 bars of pressure
Falling from a jet ski exerts between 5 and 10 bars

If you notice mist under the crystal or signs of oxidation on the dial, take your watch to a watch professional without delay.

Also read in the Glossary: Water-Resistant