Chimera tyrant

Amphibious.  The dragon turtle can breathe air and water.

Multiattack.  The chimera makes three attacks: one with its Turtle bite, one with its Beak, and one with its worm bite. 

Turtle bite. Melee Weapon Attack: +13 to hit, reach 5 ft., 1 target. Hit: 26 (3d12 + 6) piercing damage. 

Beak. Melee Weapon Attack: +13 to hit, reach 5 ft., # target. Hit: 27 (4d8 + 6) piercing damage. 

Worm bite. Melee Weapon Attack: +13 to hit, reach 10 ft., 1 target. Hit: 22 (3d8 + 6) piercing damage. If the target is a Large or smaller creature, it must succeed on a DC 19 Dexterity saving throw or be swallowed by the worm head. A swallowed creature is blinded and restrained, it has total cover against attacks and other effects outside the worm, and it takes (6d6) piercing damage at the start of each of the worm's turns.
If the worm takes 30 damage or more on a single turn from a creature inside it, the worm must succeed on a DC 21 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the worm. If the worm dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 20 feet of movement, exiting prone.

Steam Breath (Recharge 5-6).  The dragon turtle head exhales scalding steam in a 60-foot cone. Each creature in that area must make a DC 20 Constitution saving throw, taking 52 (15d6) fire damage on a failed save, or half as much damage on a successful one. Being underwater doesn't grant resistance against this damage.