Krdytkyu

In a campaign I'm running, I have players who want to attack an enemy ship. Their current plan is to use Water Breathing and swim along the bottom of the body of water the ship is in to get under the ship, then start attacking to put some holes in it and try to sink it.

I'm trying to figure out how to run the encounter. From DMG (p.119), ships have a damage threshold that must be exceeded in order to deal damage, and they have an AC. From the PHB, underwater combat places the attacker at a disadvantage if they aren't using certain weapons (dagger, trident, etc).

Thing is, the HP of the ship in the DMG is probably representative of the whole thing.
Is there any published material for the mechanics behind putting a hole in the hull of a ship, or what that does to a ship?

I recommend reading the Unearthed Arcana: Of Ships and the Sea

It has many good examples, giving Stat Blocks, combat, and other useful information you might seem important.

For example, the sailing ship entry says this:

Hull

Armor Class 15

Hit Points 300 (damage threshold 15)

that mean that unless a blow does 15 or more HP, the damage isn't enough to reduce ship hit points.

Damage Threshold

If a ship component has a damage threshold, that threshold appears after its hit points. A component has immunity to all damage unless it takes an amount of damage equal to or greater than its damage threshold, in which case it take damage as normal. Any damage that fails to meet or exceed the damage threshold is considered superficial and doesn’t reduce the component’s hit points.

As this is play test material, you may with to work with this and adapt it once you see how it plays out at the table. Also remember that if people are damaging the ship, there is a chance that someone on the ship notices.

There are not. That means it's time to improvise.

So what you have here is a unique encounter. That's great! Those can be a lot of fun, but they require a bit of improvisation.

First, figure out what rules do apply. You know that the underwater fighting rules apply. You know that the AC is 15, and no hit that deals less than 15 damage is going to matter at all. Okay. Cool. Now figure out how you want the encounter to play out for the characters. It sounds like you're aiming for a stealth encounter of sorts, so you're going to want to figure out what things can alert the crew, what things might draw other attention, and what the crew and/or other attention can do about it. Just as a rough example...

• Give the ship some number of "flood points". This indicates how much water it can take on before it sinks.


• Have some function by which damage done to the hull translates to flood points per round.


• Have something about how dealing damage to the ship in various ways can alert the crew (from the smashing noises). You can go into as much or as little detail as you want to here - either different parts of the crew get alerted at different times or they all get alerted at once or whatever. Similarly, various amounts of flooding would be likely to alert the crew (they'll notice that the ship is sinking) and possibly some other stuff.


• Figure out what the crew can do to try to thwart the PCs once alerted. (like, say pumping out water, trying to fix the holes, trying to attack, calling for help, moving the boat, or whatever). Put in some rules for those things.


• Figure out if there are any sea critters nearby that might be attracted to the noise and need to be fought off (which might make its own noise).

...something like that. You can make it more or less complicated and more or less challenging as you like depending on how much of a big deal you want this to be, and what kind of an encounter would work well with your players. When you're figuring out levels of challenging, remember that the first challenge is going to be dealing at least 15 damage while flailing about underwater in the first place. Also, as always, try to make sure that everyone who comes along on this underwater adventure has at least something useful they can do.