Bottom Treader. The swarm 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.   Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a Small humanoid. The swarm can't regain hit points or gain temporary hit points.   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.