On Ships and the Sea

Ships in general

If a ship's actions in combat matter, the ship rolls its own initiative, and takes its own actions, per the stat block. The number of crew working aboard the ship impacts how many actions it has. Note that any crew members who die, fall unconscious, work to repel boarders, etc., stop counting. Also, each of the ship's actions can only be taken once per turn.

Also, note that the ship components are individually targetable, have their own AC, etc. They also may have a Damage Threshold; anything that deals at least that much damage, is as normal. Anything that deals even one point less, deals no damage at all.

Lastly, note that most ships' number of actions is based on how many crew they have.

Your Ship

General Stat Block

The base D&D 5e stat block for a sailing ship is here. Two problems arise, though. First, your ship has no weapons. Second, the map is inaccurate; Your ship has a tiller instead of a wheel, is lacking the Lower Deck/Hold split, and instead has a proper bilge level. The basic map for your ship is here instead, and I've reproduced a more-accurate version of the stat block, below.

Ship Stat Block

Serenity

Gargantuan Vehicle (100ft. by 20ft.)

Creature Capacity 4 officers, 11 crew, 0 passengers*

Cargo Capacity 100 tons (200,000lbs)

Travel Pace 5 miles per hour (40 miles per shift, 120 miles per day)*

STR20(+5)

DEX7(-2)

CON17(+3)

INT0(-)

WIS0(-)

CHA0(-)

Damage Immunities poison, psychic

Condition Immunities blinded, charmed, deafened, exhaustion, frightened, incapacitated, paralyzed, petrified, poisoned, prone, stunned, unconscious

Actions

On its turn the ship can take actions based on the number of crew and officers combined:

20+ Three actions
10-19 Two actions
3-9 One actions
0-2 No actions

Move The ship can use its tiller to move with its sails.

Fire Cannons The ship can fire its cannons.

Fire Mangonels The ship can fire its mangonels.

Components

Hull

Armour Class 15

Hit Points 300 (Damage Threshold 15)

Control: Tiller

Armour Class 18

Hit Points 50

Move up to the speed of one of the ship's sails, with one 90-degree turn. If the tiller is destroyed, the ship can't turn.

Movement: Sails

Armour Class 12

Hit Points 100

Speed(Water)*

Into the wind: 15ft.
With the wind: 60ft.
Otherwise: 45ft.

Weapon: Cannon (2)

Armour Class 19

Hit Points 75 each

Ranged Weapon Attack: +6 to hit, range 600/2,400 ft., one target.
Hit: 44 (8d10) bludgeoning damage.

Weapon: Mangonel

Armour Class 15

Hit Points 100

Ranged Weapon Attack: +5 to hit, range 200/800 ft. (can't hit targets within 60 ft. of it), one target.
Hit: 27 (5d10) bludgeoning damage.

Serenity Floorplan

Travelling

The hard minimum crew for your ship to do anything is three people. This means that, in order to travel long distances continuously, you'll need nine people aboard with Vehicles(Water) proficiency running eight-hour shifts. You can push harder, but every hour past eight that the same person works in a day, counts as a Forced March.

Combat

In combat, it's presumed that "All Hands On Deck!" will be called, and the entire crew will contribute. In addition, any player can choose to help crew the ship instead of taking their own turn. This helps gets your total crew size up for more actions, if you're nearly at enough crew to have another action. It also helps in case enemy attacks start injuring the crew; operating at a skeleton crew of three would mean any injury stops the ship dead.

Officers And Crew

Officer Positions

Ghosts of Saltmarsh offers the following officer positions:

Captain (Abe): The captain issues orders. Useful abilities are Intelligence and Charisma, and useful proficiencies are Vehicles(Water), Intimidation, and Persuasion.
First Mate (Dripp): Supervise, encourage, and discipline the crew, keeping morals and efficiency high. Useful ability is Charisma, and useful proficiencies are Intimidation and Persuasion.
Bosun (Abe): The bosun manages the boat, giving technical advice to the captain, and leades repairs and maintenance. Useful ability is Strength, and useful proficiencies are Athletics and Carpenter's Tools.
Quartermaster (S.A.M.): The quartermaster handles navigation, plotting the ship's course at sea. Useful ability is Wisdom, and useful proficiencies are Navigator's Tools and Nature.
Surgeon (Reese): The ship's surgeon looks after sanitation, preventing disease spread, and healing injuries. Useful ability is Intelligence, and useful proficiencies are Herbalism Kits and Medicine.
Cook (S.A.M.): Works with limited and highly-salted ingredients to keep the crew fed. Useful ability is Constitution, and useful proficiencies are Brewer's Supplies and Cook's Utensils.

Officer Abilities

Three positions (Captain, First Mate, and Bosun) have the authority aboard a ship to give special orders during combat, as an action. The first order is "Take Aim". One weapon, within ten feet of the officer, gets advantage on its next attack. The other is "Full Speed Ahead". The ship's speed increases by 1d6*5 feet. If multiple officers use this, only the highest applies. Both orders last until the end of the ship's next turn.

Crew Members

Crew members each use the Commoner stat block, if it ever becomes relevant. Their one difference is that all of them are considered to have Vehicles(Water) proficiency. Crew members are a Skilled Hireling per the PBH, costing 2gp/day each. Officers will be handled on a case-by-case basis. Conveniently, the Council has given you a crew of eight for a year, which puts it at the same eight crew and four officers that the smugglers had. This does limit the ship to two actions in combat, or one if the officers are engaging in combat, but the ship only has one action, since it's currently unarmed, so that doesn't really matter. This also means that the ship is exactly at the same number of officers and crew as before, so there's currently exactly the correct number of berths.

Ghosts of Saltmarsh uses the idea of a Crew Quality Score, which is based on how the crew is treated, and goes up or down over time. The score starts at +4, and ranges between -10 and +10. Per the book, "It decreases as a crew takes casualties, suffers hardship, or endures poor health. It increases if the crew enjoys high morale, has good health care, and receives fair leadership." Based on your treatment of the crew so far, their Crew Quality Score is currently +6.