Gargantuan Dragon, Neutral
Traits
Amphibious. The dragon can breathe air and water.
Actions
Multiattack. The dragon makes three Bite attacks. It can replace one attack with a Tail attack.
Bite. Melee Attack Roll: +13, reach 15 ft. Hit: 23 (3d10 + 7) Piercing damage plus 7 (2d6) Fire damage. Being underwater doesn't grant Resistance to this Fire damage.
Tail. Melee Attack Roll: +13, reach 15 ft. Hit: 18 (2d10 + 7) Bludgeoning damage. If the target is a Huge or smaller creature, it has the Prone condition.
Steam Breath (Recharge 5–6). Constitution Saving Throw: DC 19, each creature in a 60-foot Cone. Failure: 56 (16d6) Fire damage. Success: Half damage. Failure or Success: Being underwater doesn't grant Resistance to this Fire damage.
Found on p.103 of MM2025 (Free Rules)