This thread is mostly a quick reference for published underwater rules from the Player's Handbook and the DungeonMaster's Guide, and any other sources as I run across them.

From the PHB:
Melee:
If you have am innate swimming speed (including temporarily through magic), things proceed as normal.

If not you have disadvantage unless using a dagger, javelin, shortsword, spear, or trident.

Ranged:
Regardless of swimming ability, all ranged weapons are limited to normal range. Most are at Disadvantage, except: a crossbow, a net, or a weapon that is thrown like a javelin (including a spear, trident, or dart).

Fire resistance:
Anything fully submerged has Resistance to Fire. This makes Fire spells fairly limited underwater.

Swimming:
Without an innate swimming ability you are required to make a DC 10 Con roll every hour or gain one level of exhaustion.

Depths of 100 feet or more require rolls every 20 minutes, and depths of 200 feet or more require rolls every 15 minutes.

An innate or magical swimming speed follows normal movement/forced march rules.

Visibility:
Depth and water clarity affect vision. Distances presume fully daylight on the surface, and nothing blocking light above you.

Clear water, bright light     60'
Clear water, dim light        30'
Murky water or no light       10'

Light sources work as normal in clear water. Murky water may inhibit them.

PHB 183: Suffocating
Basically you can hold your breath 1+Con modifier in minutes. Even with a negative con modifier you can hold it 30 seconds.

After that you start suffocating. You have rounds equal to your Con modifier left at which point your hits are at 0, and you're dying.

Note that speaking, including casting a spell with a somatic Component, means you have quit holding your breath and have skipped straight to suffocating Yes, this is an interpretation, which might be considered a house rule. Keep in mind that speaking require exhaling, so you HAVE just quit holding air in your lungs.
This message was last edited by the GM at 22:45, Tue 31 Aug 2021.

That is pretty much it for the DMG and PHB. Ghosts of Saltmarsh is mostly ship travel. This means I'll likely have to house rule anything not covered as they come up. Please be aware this may happen, and the rules may not be what you are hoping for/prepared for.
This message had punctuation tweaked by the GM at 00:14, Fri 17 Dec 2021.