Crustaceanfolk

Mammals are suckling babes, newcomers to this world. Elves? Dwarves? Humans? Sheepfolk? Bah. The seas are 70% of the Earth's surface, and it is there that the crustaceans have ruled supreme since the dawn of time. To them, the land is but a curiosity - or a territory to be annexed.

1. Crabfolk

Reroll CON. You have disadvantage on any checks that need fine manipulation. Pick or roll a subspecies:
1. Coconuts: You're the size of a horse and can be ridden like one.
2. Fiddlers: Can wield a 2-handed weapon or other bulky item in their Big Claw.
3. Horseshoes: 2-in-6 for blood to automatically purge toxins and diseases at beginning of day.
4. Spiders: Your long spindly limbs double your reach.

Crabs have existed almost unchanged since the beginning of time, and they take great pride in this. The many subspecies of crabfolk are quite contentious over which is the Ideal Crab, and so the Ideal Creature. Entire crabby religions have sprung up around these debates. They look like a regular crab, just scaled up to human-size (or larger).

(and how are you today, mr. giant enemy crab)

2. Krillfolk

Reroll WIS. You can fit through spaces down to 1 cm wide, but you take double damage from area attacks.

In the deepest abysses, your people learned to fight back against the whales that consume you by the tonne. By clumping together into greater forms, krill form intelligences vast and cyclopean. You are one such emergent sapience, of a krillion krill in a roughly humanoid mass. You're small for a krillfolk, for the oceanic trenches hold krillfolk titans with minds comparable to gods.

(krillfolk look like this, but vaguely humanoid)

3. Lobsterfolk

Reroll STR. Whenever you gain a level, molt and gain +1HP, but you are absolutely delicious and everyone knows this. You will be the first target in fights.

Royalty among many crustacean communities, delicacies ashore. They garb themselves in gems and precious metals, inlaid into their shells as marks of prestige. Lobsterfolk live long and grow strong, their many molts toughening their hides and thickening their muscle. It's a shame that that only makes them tastier. One in every thousand lobsterfolk is blue, instead of their usual orangy-brown, and these are often cast away to the surface as pariahs.

(a lobsterfolk just after molting and unadorned of their traditional gem-studded garb)

4. Peacock Mantis Shrimpfolk
Reroll DEX. You can make an incredibly loud unarmed attack for d12 damage [level] times per day, but your bright patterning stands out and makes it quite difficult to hide or blend in, even in the dark.

Paragons among the shrimpfolk, the peacock mantids are artisans and warrior-monks of the ocean floor. Their manyfaceted eyes let them see brilliant colors, and they use that ability well to weave beautiful kelp tapestries and forge stunning stained-glass sculptures. They also have mean right hooks, their supersonic punches creating cavitation bubbles that pop with a bang that can be heard for miles around.

(a shrimp is you)

5. Exiguan (Lousefolk)

Reroll CHA. You are a parasite that replaces another mortal's tongue and puppets them. Roll another race for your initial host, use their STR/DEX/CON and get their racial benefit/drawback. You can take a new host from any incapacitated living creature that's got a tongue around your size (yes cows, no giants). This erases the host's mind. If anyone realizes you're a horrific bodysnatching parasite, they're going to grab their buddies and try to kill you, out of perfectly reasonable fear. Anyone who sees your host's mouth open can tell you're in there. If you're separated from your host, you have STR/DEX/CON 1.

The proud and mighty Exiguan Empire rules the waves, according to them. If there is indeed such an empire, it is situated far from the other aquatic powers, and so self-sufficient it sends no traders, only a few isolated agents to explore and learn from other races. Underwater, exiguans are seen as diplomatic and friendly, if eccentric. Exiguan shopkeeps sell exotic gems and semi-magical trinkets, while few courts lack exiguan facilitators to smooth over disputes behind the scenes. On the surface, they are a horrifying legend, running every conspiracy and hidden behind every mask. Some particularly paranoid rulers ban garb that obscures the mouth, but that's clearly ridiculous. The few scattered reports of exiguan infiltrators are superstition, no more than exaggerated folktales. Right?

[IMAGE REDACTED BY ROYAL DECREE]

6. Barnaclefolk
Reroll INT. You have natural plate armor; whoever is carrying you has the same encumbrance as plate but no benefit. You're immobile, though you do have tendrils that can hold and manipulate items.

Barnaclefolk are introspective by nature, and perhaps out of necessity, as being sessile doesn't lend itself well to adventurous exploration. Yet some curious barnacles pay servants to wear them like backpacks, and carry them to other realms. They have reputations as knowledgeable scholars, creating elaborate metaphysical theories about the nature of the universe - all myopically restricted by their sessile lives.

(you, except someone's carrying you around on their back.)

n.b. I assume that all of these races are amphibious and can breathe equally well on land and underwater. If that's too powerful for your game, either don't allow them, or make them use a magical airbreathing spell/item.

(your game, now)