Multiattack. The wyvern makes two attacks or one with its breath weapon.

Bite. Melee Weapon Attack: +7 to hit, reach 10 ft., one creature. Hit: 11 (2d6 + 4) piercing damage.

Tail. Melee Weapon Attack: +7 to hit, reach 15 ft., one target. Hit:  13(2d8+4) bludgeoning damage.

Geyser Breath (Recharge 5-6).  The Serpent exhales refreshing or frigid water in a 30-foot line that is 5 feet wide The target must make a DC 15 Dexterity saving throw, taking 24 (7d6) cold damage on a failed save, or half as much damage on a successful one. Or the dragon will instead create a refreshing current, healing instead of dealing damage.