The canon rules for underwater adventuring are rather short and specialized to travel and combat. As other issues come up, I'll likely have to house rule things. Tracking, for instance, is likely to be rather hard as swimmers don't leave foot prints.

As such issues come up, I'll address them here. Please be aware that what I come up with may differ from what you are hoping for or prepared for. And the rules I come up with may not survive playtesting and may need to be changed. This is the point of this campaign, after all: To get working rules for long term underwater adventuring.
This message was last edited by the GM at 16:40, Fri 12 May 2023.

I had not planned on any house rules to start, but there are issues with spell casting and spoken and material components.

Material Components:
Truthfully, an aquatic environment makes this an actual issue. Live grasshoppers, plants only found on the surface, bits of dust, fluid, etc all are problematical underwater. The solution for most is easy: A Focus. Keep in mind that wands and staves, as wood, are prone to issues when waterlogged, and could float away when let go. Opt for Orbs, crystals, holy symbols (that aren't wooden), relics, totems, etc.

Even then some spells, such as Bless might be impossible underwater, although there may be creative workarounds (usually involving a surface artificer).

Somatic Components:
Aquatic Races are fine here, but the Fathomless Warlock makes it clear that Speaking underwater is different from merely breathing underwater. For non Aquatic types, this makes Vocal component spells difficult for surfacers. There ARE some magical workarounds: the Helm of Underwater Action makes an air bubble, for instance.
This message was last edited by the GM at 06:45, Sat 05 Aug 2023.

This is an issue for air breathers, one that I'm not sure how to resolve simply, but but the 6th level Fathomless Warlock ability (From Tasha's) makes it clear this is  separate form Water Breathing, although Water Breathing is likely a prerequisite.
This message was last edited by the GM at 06:48, Sat 05 Aug 2023.

Truthfully, many just won't work. I have workaround for coral/shell armor and weapons, but it is harder than on the surface.

This list is for those trained/raised underwater, of course. Those trained raised on the surface follow normal rules and may need special training to adjust.

What just doesn't work:
Alchemy tools
Brewer tools
Calligrapher's tools
Cook's utensils
Glassblower's tools
Painter's supplies
Potter's tools
Smith's tools
Playing Cards
Herbalism
Any Musical Instrument

What needs special handling:
-Carpenter's tools (Used with Animal Handling for Coral work)
-Mason's tools (Used with Animal Handling for Coral work)
-Tinker's tools (most Xanathar's applications impossible under water, but with Animal Handling, could be used to grow coral patches on some items. it'll take extra time)
-Weaver's tools (not a lot of fabric or thread down there, but not impossible)
-Woodcarver's tools (Used with Animal Handling for Coral work)
-Disguise Kit (possible that it will have limited use but the makeup is ruined)
-Forgery Kit (possible that there is some sort of aquatic documentaion, probably using leather or stone/shell/coral work)
-Three Dragon Ante (ie: 3 card Monte or a Shell game)
-Poisoner's kit

What DOES work:
Leatherworking is fine, as is Cobbling (although pretty much unnecessary and, for some, unwanted)

Leatherworking alone is used to make Leather and Studded Leather (using coral studs), and Hide. Some know the tricks of making shell based Scale armor, as well.

With Animal Handling, it can make Breastplates from giant Shells and grown Coral.

Other Armor types are unlikely, especially if mail based.

Shields are basically Large Shells with leather straps.

Woodcarving combined with Animal Handling can be used to make many Coral based piercing melee weapons. Spines from some sea creatures may be useful as well. This is also the skill for bone carving.

Weaving alone can make nets for any net using species. Finding the appropriate fibers may require a bit of searching, of course.
This message was last edited by the GM at 00:35, Wed 01 Sept 2021.

Weapons, armor, and tools have been covered. Most of the rest is common sense:

Avoid:
Wood
Liquids (including lamps and lamp oil)
Fabrics

Clothing and containers (sacks, pouches, etc) are made from the leather of aquatic creatures.

Some wooden items might be available from other materials (spines for bolts, for instance), others might not (foci, for instance).

Avoid component pouches, and most stored food and water.

If you want a torch, pay for a Continual Flame spell on an item when you can afford it.

Much of the rest will be common sense. If you're a surfacer I'll point out your mistakes after the game starts. If aquatic raised, before. You can ask first, but on a per item basis, not the whole list. Even then, I may not tell you, depending on circumstances.
This message was last edited by the GM at 00:11, Fri 17 Dec 2021.