Hold Breath. The hydra can hold its breath for 1 hour.

Multiple Heads. The hydra has three heads. While it has more than one head, the hydra has
advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious. Whenever the hydra takes 15 or more damage in
a single turn, one of its heads dies. If all its heads die, the hydra dies. At the end of its turn, it grows a head for each of its heads that died since its last turn, unless it has taken fire damage since its last turn. The hydra regains 5 hit points for each head regrown in this
way.

Reactive Heads. For each head the hydra has beyond one, it gets an extra reaction that can be used only for opportunity attacks.

Multiattack. The hydra makes as many bite attacks as it has heads.

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 5 (1d6 + 2) piercing damage.