Captain Holt

Amphibious. Captain Holt can breathe air and water.

Evasion: If Captain Holt is subjected to an Effect that allows it to make a Dexterity saving throw to take only half damage, he instead takes no damage if it succeeds on the saving throw, and only half damage if it fails.

Sea legs: The Captain has advantage on Strenght and Dexterity skill checks made whilst standing on a waterborne vehicle (as described on page 157 of the players handbook). 

Captaining

Spectral Cannon. Ranged Spell Attack: +8 to hit (+12), range 240 ft., one target. Hit: 13 (1d10 + 8) bludgeoning damage.

Spectral Net. Ranged Spell Attack: +8 to hit (+12), range 60 ft., one target. Hit: A creature hit by a net is Restrained until it is freed. A net has no Effect on creatures that are formless. A creature can use its action to make a DC 15 Strength check, freeing itself or another creature within its reach on a success. Dealing 5 slashing damage to the net (AC 15) also frees the creature without harming it, Ending the Effect and destroying the net.

Swerve. Captain Holt takes the wheel, swerving the ship left or right. Each creature in the sea train must make a DC 15 Dexterity saving throw or be thrown to the opposite side of the ship, taking 1d6 damage per 5 feet travelled.

Plunge. Captain Holt takes the wheel, plunging the ship underwater for 1d4 turns. While underwater, all creatures on the ship cannot talk or cast spells that use verbal components, ranged weapon attacks are made at disadvantage, fire damage is halved, and the train counts as difficult terrain for those without a swim speed.

Boarded

Multiattack.  The captain makes 3 attacks. One with the hand crossbow and two with it's cutlass. 

Saber of the Savage Seas. Melee Weapon Attack: +7 to hit, reach 5 ft., 1 target. Hit:  (1d6 + 5) [Slashing] damage. On a Critical Hit (20) deals an additional 1d10 psychic damage. (Magical Weapon, ignores disadvantage on attacks imposed by being underwater.)

Cursed Bolt. Ranged Spell Attack: +3 to hit, range 30/60 ft., 1 target. Hit:  (1d12 ) [necrotic] damage.

Shared Suffering. As a Reaction, the Captain Holt may harness the pain of its suffering while imprisoned within the Hells. When hit by another creature, the captain may unleash an unearthly howl and anything within 5 ft of the captain must succeed on a DC 15 constitution save or be paralyzed for 1 round. This Reaction may only be used 2 times per short rest.

Spectral Anchor (Costs 1 Action). Captain Holt manifests a Spectral anchor on one creature within 30 feet. At the beginning of their turn, the creature must make a DC 15 Strength saving throw or have their movement set to 0 for their turn. All spectral anchors disappear at the start of Captain Holt's turn.