Undead Nature.
The drowned one doesn't need to eat, breathe, drink, or sleep.   Undead Fortitude.
If damage reduces the zombie to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the zombie drops to 1 hit point instead.   Putrescence. (only applicable to Putrid Variety)
When the drowned one is damaged with a slashing or piercing weapon, it releases a wave of putrid stench. All creatures in a 10-ft radius must make a DC12 CON save or become poisoned. While poisoned this way, the creature must spend its action gasping for breath, unless the creature doesn't need to breathe air. This effect lasts 10 mins, with the creature making a save at the end of each of its turns. A successful save clears the effect.