Armour's durability decreases if you take fall damage AND drowning damage.

*Steps to Reproduce-*

1. Equip leather shoes with full durability.
2. Fall down from a height.
Result- Durability decreased.

1. Equip leather shoes with full durability.
2. Go underwater and open your armour screen.
3. Take damage until you die.
Result- As you take damage, your shoes take damage too.