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.)
Best Answer
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."
* - I didn't say it was a good joke.