The constant approach of aquatic races, especially those portrayed by the 'world most popular role-playing game' tend to make a race that is very much like their surface cousins, just they have blue skin and gills attached to their necks. I've never seen anyone play them or go out of their way to advocate much for them, and my personal opinion is that while I have nothing against them, I believe the deeps of the ocean should be reserved for monstrous things like Aboleths, Sahuagin, and Kraken. I guess merfolk may be one of the friendlier races I like to see dwell in my ocean, not a blue-green Dwarf with gills coming out of his neck. So! I wanted to make a few water dwelling races.
-5e: All of these 'groups' are proficient in swimming.
-Mini Six: Each of these races begin play with a Specialty die in Athletics (Swimming).
-5e: Use suffocating rules (PHB 183) to simulate drowning. An aquatic character has trained their body and gain Con Modifier + 3 in minutes before drowning with a minimum of 1 minute.
-Mini Six: May hold their breath for a number of minutes equal to their Might Die with no bonuses + 2 extra rounds before fearing drowning.
Sea/Water-based race groups
Brief: Based out of remote islands, the Sea Clan is very different from popular conception of Dwarves. They tend to be relatively thinner and tanner compared to their underground dwellers. The Sea Clan are often merchants either moving goods such as rare fruit or glass beads to nurturing and harvesting oysters fer pearls. Their rivals are Corsair Orcs whom the two are known to raid each other for spoils and rights to fish certain plentiful regions of the Azure Sea. Sea Clan Dwarves tend to feud with Pirates who bring trouble and lawlessness to the region.
Brief: Found along the coasts of either ocean or river, Shore Gnomes are groups of Gnomes that known to fish and trap the shallows for things such as crab or lobster. With small boats they like to paddle to coral reefs to selectively harvest certain corals for alchemical or magical reagents. Such an act requires either incurring the wrath or making a deal with merfolk who patrol the area. Shore Gnomes are noted for a willingness to build or at least maintain lighthouses. A Shore Gnome with the distinction as a Keeper takes their job very seriously as no Gnome wants the reputation as being a bane to ships. Shore Gnomes tend to be a quiet and suspicious lot, they've seen one too many haunted ships try to either land or take over lighthouses to be as jovial as their kin who stuck to land.
Brief: Orcs of the sea start out on island communities and are often raised under the mantra that either cruel, dark things lurk beneath the waves- so you better sacrifice something to appease them, or that the sea is controlled by a beautiful bitch queen who needs to be placated. Either way, Orcs know to drink and eat hearty, spend the treasure when you get it, and the occasional sacrifice of captured slaves or animals wouldn't hurt either. Corsair Orcs (and Half-Orcs) are often found in the company of other pirate gangs in the region. There are few instances where ship is crewed entirely by Orcs, those that are can be highly aggressive and may accept little ransom, preferring to take the ship and capture slaves to either sell or sacrifice to whichever patron the crew chooses to follow.
Brief: Also known as River Halflings, the Fisherfolk typically dwell along rivers, in swamps, or marshes. Any place where the fishing is good. Over time and training these Halflings have an instinctive sense of finding the perfect spot and the patience needed to wait and draw in the perfect catch to eat or sell. A lot of these folk train their bodies for extended dives when they wish to search the river bottom or the possibility that a large fish drags them under. Their main rivals are often Saurians for control of these territories. Fisherfolk Rangers & Druids like to even the score against their rivals by taming alligators and crocodiles as their mount / animal companion.