Size Gargantuan Vehicle (165 ft. by 25 ft., 810,000 lb.)
Creature Capacity 0 crew, 200 passengers
Cargo Capacity 300 tons
Travel Pace 5 miles per hour (120 miles per day)

On its turn, the mimic can take 3 actions, choosing from the options below.

Batten Down. The mimic can activate or deactivate its batten down hatches.

Drop/Weigh Anchor.  The mimic can drop or attempt to raise its anchor.

Fire Ballistas. The mimic can fire its harpoon guns.

Fire Mangonels. The mimic can fire its mangonels.

Haul In. The mimic can pull each target grappled by one of its harpoon guns 30 feet closer, and each target grappled by one of its pseudopods 10 feet closer.

Jaws. The mimic can use its jaws.

Move. The mimic can use its helm to move with its pseudopods or sails. As part of this move, it can use its naval ram.

Pseudopods. The mimic can attack with its pseudopod oars.

Repair. The mimic can fully restore 1 of its destroyed components.

Hull

Armor Class 17

Hit Points 500 (damage threshold 20)

The hull is the mimic's main body and is synonymous with the mimic.  Whenever one of the mimic's ship components is destroyed, it takes damage equal to half hit point maximum of the destroyed component, which cannot be reduced in any way.  The mimic's hull has the following traits.

Shapechanger. The mimic can use its action to polymorph into a galley, sailing ship, or warship by drawing its eyes, pseudopods, and teeth into its body, or back into its true form by extending them again.  It can also polymorph its interior into fully furnished cabins and compartments, or revert such furnishings into its true, amorphous form. Its statistics are the same in each form. Any creatures, equipment, or objects it is carrying aren't transformed. It reverts to its true form if it dies. If cleaned out thoroughly, the mimic’s remains can be converted into a vessel.

Adhesive. The mimic adheres to anything of its choosing that touches it. A creature adhered to the mimic is also grappled by it (escape DC 26). Ability checks made to escape this grapple have disadvantage.  The mimic can excrete, cease excreting, or dissolve its adhesive at will.

False Appearance. While the mimic remains motionless, it is indistinguishable from an ordinary vessel.

Grappler. The mimic has advantage on attack rolls against any creature or object grappled by it.

Regeneration. The mimic regains 30 hit points at the start of each of its turns if it has at least 1 hit point.  Each time the mimic regenerates, each of its ship components regains 10 hit points.

Control: Helm

Armor Class 18

Hit Points 50

The mimic moves up to the speed of one of its movement components, with one 90-degree turn. If the helm is destroyed, the mimic can't turn.

Control: Anchor

Armor Class 19

Hit Points 50 (damage threshold 10)

It takes the mimic one action to drop its anchor and 3 actions to raise it.  When the anchor is dropped, it falls up to 300 feet downwards in a 10-foot-radius cylinder towards a point directly beneath the bow. Each creature or object in the area when the anchor is dropped must succeed on a DC 15 Dexterity saving throw or be struck by the falling anchor, taking 44 (8d10) bludgeoning damage.  If the anchor lands on a solid surface, the mimic becomes moored in place until the anchor is raised again. While moored, the mimic's speed becomes 0, and it can't benefit from any bonus to its speed. Whenever an effect forces the mimic to move, it can make a DC 21 Strength saving throw. On a successful save, the mimic is not moved, or moves only half as far on a failure.

Movement/Weapon: Pseudopod Oars (12)

Armor Class 15

Hit Points 20 each; -5 ft. speed for each component destroyed

Speed (water) 40 ft.

The mimic's pseudopods can be used to move as well as to attack.  When the mimic takes an action that moves it with its pseudopods, any creature or object in the water within 10 feet of it at any point of the move must make a DC 10 Dexterity saving throw.  On a failure, a target is subjected to its Adhesive trait and grappled by one of the mimic's pseudopods.  When the ship takes an action that uses its pseudopods as a weapon, each pseudopod can make one of the following attacks.

Melee Weapon Attack: +18 to hit, reach 30ft., one target. Hit: 36 (4d12 + 10) bludgeoning damage, and the target is subjected to the mimic's Adhesive trait.  Until this grapple ends, a Gargantuan target's speed isn’t reduced, but it can't move farther away from the mimic; a Huge or smaller target is restrained until the grapple ends. Each of the mimic's pseudopods can grapple one target.

Fling: One Huge or smaller object held or creature grappled by one of the mimic's pseudopods is thrown up to 60 feet in a random direction and knocked prone. If a thrown target strikes a solid surface, the target takes 3 (1d6) bludgeoning damage for every 10 feet it was thrown. If the target is thrown at another creature, that creature must succeed on a DC 26 Dexterity saving throw or take the same damage and be knocked prone.

Movement: Sails

Armor Class 12

Hit Points 100; -15 ft. speed per 25 damage taken

Speed (water) 45 ft.; 15 ft. while sailing into the wind; 60 ft. while sailing with the wind

Weapon: Harpoon Gun (4)

Armor Class 15

Hit Points 50 each

Ranged Weapon Attack: +18 to hit, range 120/480 ft., one target. Hit: 26 (3d10 + 10) piercing damage, and the target is subjected to the mimic's Adhesive trait. Until the grapple ends, a target's speed isn’t reduced, but it can't move farther away from the mimic.  Each of the mimic's harpoon guns can grapple one target.

Weapon: Jaws

Armor Class 17

Hit Points Shares the mimic's hit points and hit point maximum

Melee Weapon Attack: +18 to hit, reach 5ft., one target. Hit: 36 (4d12 + 10) piercing damage plus 26 (4d12) acid damage.  If the target is a Huge or smaller creature, it must succeed on a DC 26 Dexterity saving throw or be swallowed by the mimic.  A swallowed creature is blinded and restrained, has total cover against attacks and other effects outside the mimic, and takes 56 (16d6) acid damage at the start of each of the mimic's turns.

If the mimic takes 60 damage or more on a single turn from a creature inside it, the mimic 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 mimic. If the mimic dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 60 feet of movement, exiting prone.

A Huge or smaller target reduced to 0 hit points by damage from the mimic's bite or while swallowed is ground to bits and dissolved in acid.  Any nonmagical items the target was holding or carrying are destroyed as well.

Weapon: Mangonel (2)

Armor Class 15

Hit Points 100 each

Ranged Weapon Attack: +18 to hit, range 200/800 ft. (can't hit targets within 60 ft. of it), one target. Hit: 37 (5d10 + 10) bludgeoning damage.  Whether the attack hits or misses, each creature within 20 feet of where the projectile lands must make a DC 15 Dexterity saving throw.  On a failed save, a creature takes 25 (10d4) acid damage and another 12 (5d4) acid damage at the end of its next turn. On a successful save, a creature takes half the initial damage and no damage at the end of its next turn.

Weapon: Naval Ram

Armor Class 20

Hit Points 100 (damage threshold 20)

The mimic has advantage on all saving throws relating to crashing when it crashes into a creature or an object. Any damage it takes from the crash is applied to the naval ram rather than to the mimic. These benefits don't apply if another vessel crashes into the mimic. 

A creature or object struck by a mimic must make a DC 20 Dexterity saving throw, taking 88 (16d10) bludgeoning damage on a failed save, or half as much damage on a successful one.

Miscellaneous: Batten-Down Hatches

The mimic releases any creature grappled by it, retracts into its shell, then shuts and locks its hatches, doors, and portholes.  While the mimic is battened down, it gains a +8 bonus to it AC against all attacks made from outside of it, and any hatches, doors, and portholes in or on its body is affected as they would be by an arcane lock spell.  While it is battened down, the mimic can't attack with its jaws or pseudopod oars, it is blinded, and it can only use its sail to move.  It can continue to take other actions and lair actions as normal.  The mimic can end this effect as an action. 

This effect ends automatically if the mimic dies, is subjected to the knock spell, or a creature swallowed by the mimic causes it to regurgitate.

Batten Down. When an attack reduces the mimic to half its total hit points or less, it can use its Batten Down action.  Once the mimic has used this reaction, it can't do so again until it finishes a short or long rest.