Oiled Body. The mizutsune has advantage on Dexterity (acrobatic) checks and Dexterity saving throws.

Multiattack. The mizutsune makes one Bite attack and two Tail attacks.

Bite. Melee Weapon Attack: d20+11 to hit, reach 5 ft., one target. Hit: 19 (3d8 + 6) piercing damage. If the target is a Large or smaller creature, it is grappled (escape DC 13). Until this grapple ends, the mizutsune can't use its bite on another target.

Bubble Prison. Ranged Weapon Attack: d20+11 to hit, range 80/320 ft., one Large or smaller creature. Hit: The creature is slick and enclosed in a bubble of shimmering force for 1 minute or until the bubble is destroyed. Nothing, not physical objects, energy, or other spell effects, can pass through the bubble, in or out, though a creature in the sphere can breathe. A creature inside of the bubble has disadvantage on attacks due to its slippery nature and takes 5 (1d10) fire damage at the start of each of its turns. The bubble can be attacked and destroyed (AC 15; hp 40; resistance to bludgeoning damage; immunity to poison and psychic damage; vulnerability to piercing damage). At the end of the mizutsune's turn all bubbles rise 15 feet higher into the air.

Slam. Melee Weapon Attack: d20+11 to hit, reach 5 ft., one target. Hit: 22 (3d10 + 6) bludgeoning damage. If the mizutsune moved at least 10 feet straight toward the target immediately before the hit, the target takes an extra 16 (3d10) bludgeoning damage. If the target is a creature, it must succeed on a DC 19 Strength saving throw or be knocked prone.

Tail. Melee Weapon Attack: d20+11 to hit, reach 10 ft., one target. Hit: 25 (3d12 + 6) bludgeoning damage.

Bubble Barrage. The mizutsune creates five bubbles around it. When it uses this action, and as a bonus action on subsequent turns, the mizutsune can hurl one bubble at a point within 120 feet of it. Each creature within 5 feet of the point must make a DC 18 Dexterity saving throw, taking 11 (2d10) fire damage and ignites on a failed save, or half as much damage and doesn't burst into flames on a successful one. Until a creature takes an action to douse the fire, the target takes 5 (1d10) fire damage at the start of each of its turns.

Scaling Stream (Recharge 5-6). The mizutsune releases a high pressure stream of water in a 90-foot line that is 5 feet wide or in a 60-foot cone. Each creature in that area must make a DC 18 Dexterity saving throw, taking 55 (10d10) fire damage on a failed save, or half as much damage and doesn't burst into flames on a successful one. Until a creature takes an action to douse the fire, the target takes 5 (1d10) fire damage at the start of each of its turns.

Bath Bomb (Recharges after a Short or Long Rest). The mizutsune forms a Huge bubble that hovers 30-feet above it, and moves with it. On its subsequent turn, the mizutsune an use an action to perform a dance, that creates a vortex around, before the bubble impacts the ground and explodes. Each creature in a 60-foot-radius, 60-foot-high cyclinder centered on the bubble must succeed on a DC 18 Strength saving throw or be pulled 30 feet towards the bubble and automatically fails its saving throw against the explosion. Each creature within 30 feet of the bubble when it explodes must make a DC 18 Dexterity saving throw, taking 90 (20d8) fire damage on a failed save, or half as much damage and doesn't burst into flames on a successful one. Until a creature takes an action to douse the fire, the target takes 5 (1d10) fire damage at the start of each of its turns.
     The mizutsune must concentrate on the bubble (as if concentrating on a spell) while it remains in the air above it. If it fails its Constitution saving throw to maintain concentration on the bubble, it explodes, dousing any flames within 60 feet of the bubble.

Enter the description for how bonus actions work for your monster here.

The mizutsune can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The mizutsune regains spent legendary actions at the start of its turn.

Attack. The mizutsune makes a Bite attack.

Encapsulate. The mizutsune uses Bubble Prison.

Bubbles (Costs 2 Actions). The mizutsune uses Bubble Barrage.

Slide (Costs 2 Actions). The mizutsune moves up to its speed, during this move it can move through other creatures without provoking opportunity attacks. Each creature the mizutsune moves through must succeed on a DC 19 Dexterity saving throw or take 20 (4d6 + 6) bludgeoning damage, be pushed up to 10 feet away and knocked prone.

Spray Burst (Costs 2 Actions). The mizutsune spurts out liquid that instantly explodes. Each creature in a 15-foot square originating from the mizutsune must make a DC 18 Dexterity saving throw, taking 16 (3d10) fire damage on a failed save, or half as much damage and doesn't burst into flames on a successful one. Until a creature takes an action to douse the fire, the target takes 5 (1d10) fire damage at the start of each of its turns.