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.)