How does combat work underwater?

Question

I've been a 5e player for the last 6 months or so and soon I'll be DMing for the first time, as short filler when our main campaign can't go ahead (for example, our paladin is on holidays).

My campaign centers around a town terrorized by an Aboleth, which the heroes have been contracted to deal with. Aboleths are smart, so unless the heroes do something interesting, it will likely try to fight in the water where it will have the advantage.

I have read through the DMG and some other online guides but there doesn't seem to be a consistent way to run this style of battle.

I understand that movement speed should be ¼ of the heroes normal movement, unless they've got some modifiers or such.

The main things I need to understand are:

• How does spell casting work underwater?
• How should breathing mechanics work underwater?

Answer 1

Firstly, you didn't ask, but movement is effectively cut in half, rather than quartered.

While climbing or swimming, each foot of movement costs 1 extra foot (2 extra feet in difficult terrain), unless a creature has a swimming or climbing speed.

So movement is ⅓ if the heroes are in difficult terrain - normal swimming is only ½.

Next, spellcasting. There are no specific rules about this, but:

Most spells require the chanting of mystic words. The words themselves aren’t the source of the spell’s power; rather, the particular combination of sounds, with specific pitch and resonance, sets the threads of magic in motion. Thus, a character who is gagged or in an area of silence, such as one created by the silence spell, can’t cast a spell with a verbal component.

It's pretty reasonable to extend this to a character underwater, so spells with verbal components are out. There doesn't seem to be anything else that would restrict underwater spellcasting, however.

Finally, breathing mechanics. The rules for this are on page 183 of the PHB, and are much simpler:

A creature can hold its breath for a number of minutes equal to 1 + its Constitution modifier (minimum of 30 seconds).

When a creature runs out of breath, it can survive for a number of rounds equal to its Constitution modifier (minimum 1 round). At the start of its next turn, it drops to 0 hit points and is dying, and it can’t regain hit points or be stabilized until it can breathe again. For example, a creature with a Constitution of 14 can hold its breath for 3 minutes. If it starts suffocating, it has 2 rounds to reach air before it drops to 0 hit points.

Answer 2

There are 5 main differences.

Movement

You spend twice your movement to swim, unless you have a swimming speed. Difficult terrain triples the movement you spend instead.

While swimming, each foot of movement costs 1 extra foot (2 extra feet in difficult terrain), unless a creature has a swimming speed.

Source here.

Weapon Combat

Usable underwater melee weapons are: dagger, javelin, shortsword, spear, trident. All other have disadvantage if you don't have a swimming speed.

Ranged weapons attacks miss beyond their normal range, and have disadvantage unless they are: crossbow, a net, or a weapon that is thrown like a javelin (including a spear, trident, or dart).

Source here.

Fire Resistance

If fully immersed, half-damage from fire.

Source here.

Spellcasting

There seems to be some debate regarding whether you can cast Verbal spells. Nothing in RaW says you can't cast them. The RAI is yes, but you start drowning, unless you can breathe underwater, in which case you have no trouble at all.

Breathing

You can hold your breath for 1+CON minutes. When you run out, you can survive for CON rounds while suffocating.

At the start of its next turn, it drops to 0 hit points and is dying.

The creature can't be stabilized or healed until it can breathe again.

Source here.