Bottom Treader. The undead cannot swim, and it sinks to the bottom of any body of water. It takes no penalties to its movement or attacks underwater. It is immune to the effects of being underwater at a depth greater than 100 feet.
Undead Fortitude. If damage reduces the undead 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 undead drops to 1 hit point instead.