How can characters know which weapons to use underwater?

Question

My 5th Edition party is gearing up to take a short trip underwater. They found a friendly spellcaster to cast Waterbreathing on them and they're going to locate a giant octopus lair. In preparation, I've made a cheat sheet of the underwater rules from the PHB and the DMG to help me run the session.

My question concerns the rules for melee and missile weapons when used underwater. Anyone not using a very small set of melee and missile weapons will have disadvantage when attacking underwater.

Do the characters automatically know that these penalties exist? (None of the PCs have a background involving the water, like a sailor, etc.) If not, what roll(s) could they make to determine if they know which weapons to use to avoid having disadvantage on attack rolls?

(Note: This is about the characters knowing which weapons to use, not the players.)

Answer 1

My tables: the characters know the underwater combat rules just as well as the players.

My interpretation--and this is just one man's thinking--is that anything in the PHB should be considered fair game for character knowledge. It's been my interpretation for decades, and has worked out well at plenty of tables.

The fundamental process is "GM narrates environment, player describes actions, GM narrates results." But we go deeper than that every time we play. We want to go deeper than that. We want there to be a world, not just the local environment. We want our characters to have histories, and backstories. This-all implies a lot of knowledge and experience on the character's part beyond just what's on the character sheet and what's been narrated at the table.

The PHB reflects all this assumed-common knowledge. At my tables the character knows her standing long jump is 6', even though she has no idea that she has a strength "score" of 12. My bard knows that he's only going to inspire someone X times per day, no matter how rousing the song. And all characters know that piercing works better than slashing or bludgeoning underwater, because they've spear-fished as kids, they've heard stories from their granddad, they've told and re-told in-universe myths and fairy tales that inculcate such knowledge. Jokes even. "Why'd the dwarf drown with his club in his hand? Because he couldn't beat his way out of a rain-barrel! With a club! Because it does bludgeoning damage!"^*

In my experience, anything less than granting characters all the knowledge in the PHB quickly turns into a game of "mother, may I?" And that's not "the world's greatest roleplaying game."

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^{* - I didn't say it was a good joke.}

Answer 2

When deciding whether to use a roll, ask yourself ...: Is a task so easy and so free of conflict and stress that there should be no chance of failure? ... If the answer ... is no, some kind of roll is appropriate. (DMG 237)

What is the information the PCs seek exactly?

1. Fighting underwater is harder than on land.
2. Projectiles do not go very far in water.
3. Weapons that are used with a thrusting motion and nets are impeded less.
4. Fire hurts things in water less.

Out of these only the third one might require any thought. Even a simple farmer could tell you the others.

Regarding thrusting weapons (and nets): Most likely someone has heard of a harpoon and nets. You do not have to be a fisherman to know what kind of tools they use. They were most likely designed to work in water, no?

If you want to be stingy you could require a roll for #3, but if they have plenty time to prepare 3 to 6 adventurers could be expected to figure it out barring special circumstances (all of them have <8 Int or something).

Answer 3

Using Background, Classes, and Ability Checks

As DM you get to decide if the players "automatically know" or if they have to learn things like this. This falls under the matter of DM style.

What is basic knowledge?

Since the characters grew up into their late teens/early 20's (if human) in this world and learned all kinds of things while growing up and learning a profession/class, you may ask yourself why anyone who is of the martial class would not know this?

Further that point, anything in the PHB can be considered as things that the characters will have learned while growing up / being trained. While the character sheet is a tabula rasa the characters themselves are not. You (the DM) can decide that they automatically know this kind of thing since it is in the PHB.

If you want to make your players work for this ...

Backgrounds and Classes

A way to assess this kind of knowledge is to look at the Classes, Professions, and backgrounds that your characters have, as well as their Skills.

Is there a Sailor, Folk Hero or Outlander (the last two have the Survival Skill) or a Soldier in your group? (Hmm, per your edit, no Sailor, but we'll press on).

• Survival includes living off the land which would include getting food from water, or underwater. That's a way for a character to already know what weapons do and don't work best on creatures in the water. You could offer advantage on an intelligence check, or just say "you remember this from" or "you learned this from" that part of your background if you choose to offer that advice rather than make the players work for it.

• Martial Classes (why wouldn't they know?) The general case for a Fighter knowing what weapons work best underwater seems pretty basic knowledge for someone whose profession is just that: combat.

  Is there a Fighter in the group? That class/profession would tend to know details like this as part of their training. A Fighter would know this kind of detail about underwater fighting ... and it's unlikely that a Barbarian, Ranger or Paladin would not know.

Ability Check

The ability score Intelligence includes memory and reasoning in its description.

Intelligence measures mental acuity, accuracy of recall, and the ability to reason. (Basic Rules p. 61)

Of the ability checks based on Intelligence (Arcana History Investigation Nature Religion) Investigation may be what you need, or just a generic "Intelligence" ability check.

Other Intelligence Checks. The DM might call for an Intelligence check when you try to accomplish tasks like the following: {snip}
• Recall lore about a craft or trade {or in this case lore about underwater combat}.

The point on weapons and underwater fighting could come easily up in a conversation with whomever cast the spell, in terms of what weapons work best underwater. (Not using a game mechanics term, but some "advice from Trido the mage...")

Answer 4

Whilst you say they have no extensive background of things aquatic, I'd at least expect them all to be able to swim. And if they can swim then everything about this should be self-evident from the characters' own experiences of the properties of water.

If none of them can swim, the players could easily cover this lack of knowledge by having their characters check out how well their equipment works underwater before they set out. It'd only take a couple of minutes to discover that maces and throwing knives are best left on the shore. Or their "friendly spellcaster" could advise them.

Even without this, anything the characters know is an issue with water on land, they should be assumed to know will also apply underwater. So archers can be assumed to know about the effects of water on bowstrings, anyone with fire-related abilities can be assumed to know it isn't going to work well underwater, and so on.