On Fish-People

I have long complained that, of all the array of fish-people and seafolk available in D&D, they're all kind of samey. Let's see, we've got...

• Merfolk are people with a fish tail
• Tritons are people who live underwater and are just kind of slightly fishy -- in earlier editions, they were at least Outsider(native)s, which is something to latch on to. (Although it does make them kind of overlap with water genasi -- but without it, they instead overlap with, like sea elves.)
• Locathah are just kind of generically fishy, although in earlier editions they kind of looked like the Creature from the Black Lagoon
• Sahuagin are just kind of generically fishy, albeit with a telepathic connection to sharks
• Kuo-toa are just kind of generically fishy, albeit also slimy

So let's see what I did with that to make it a little less boring (and also options for player characters, because my setting is so heavy on seafolk)...

Merfolk

Species Traits

• Ability Score Increases: Your Dexterity score increases by 2, and your Charisma score increases by 1.
• Type: Your type is Humanoid.
• Size: Your size is Medium.
• Speed: You have a land speed of 10 feet and a swim speed and a swim speed of 40 feet. You begin play with a free Combat Wheelchair, which grants a land speed of 25 feet.)
• Amphibious: You can breathe both air and water.
• Versatility: You gain one free skill proficiency of your choice and one free tool proficiency of your choice.
• Languages: You can read, write, and speak Common and Aquan.

Triton

In previous editions, tritons were native outsiders. In mine, they're still immigrants from the Hellish Plane of Water. So...

Species Traits

As in Mordenkainen Presents: Monsters of the Multiverse, except:

• You have the Humanoid and Elemental types. If any favorable effect would affect either of your types, it affects you. If any unfavorable effect would affect one of your types but not the other, you can be affected by it but you have advantage on saves against it. If any effect would affect neither of your types, you are immune to it. If any effect would affect both of your types, you do not gain immunity or resistance to it from your types. 

Sahuagin

Notwithstanding recent efforts to make them, like, green and lizardy, Sahuagin can just be shark-people.

Species Traits

• Ability Score Increase: Your Strength score increases by 2, and your Wisdom score increases by 1.
• Type: Your type is Humanoid.
• Size: Sahuagin are larger than humans, ranging from 6 to 7 feet tall. Your size is Medium.
• Speed: You have a base land speed of 30 ft, and you have a swim speed equal to your land speed.
• Superior Darkvision: Within 120 feet, you can see in dim light as if it were bright light, and in darkness as if it were dim light. You can't discern color in darkness, only shades of gray.
• Limited Amphibiousness: You can breathe both air and water. However, you cannot receive the benefits of a Short or Long rest unless you are fully submerged in water for the entire duration of your rest.
• Frenzy: On your turn when you reduce a creature to 0 hit points, you can use a bonus action to move up to your speed and make an additional weapon attack against another creature you can target.
• Shark Telepathy: You can telepathically speak to any shark within 120 feet of you.
• Claws and Bite: Your claws and bite are natural weapons, which you can use to make unarmed strikes. If you hit with them, you deal slashing (claws) or piercing (bite) damage equal to 1d4 + your Strength modifier, instead of the bludgeoning damage normal for an unarmed strike.
• Languages: You know Common and Sahuagin.

Kuo-Toa

Kuo-toa shall be anglerfish, complete with anglerfish sexual dynamics: 

• Males are Tiny Beasts with 1 Intelligence (using the Quipper statblock).
• Females are Medium Humanoids (using the Kuo-Toa statblock, or the species traits here).

Biologically female Kuo-Toa have six genders (Up, Down, Charm, Strange, Top, and Bottom), which they borrowed from the sexual hexamorphism of the Aboleths they chill with. It's not well-understood by surface-dwellers how romantic combinations of these genders works.

Species Traits

• Ability Score Increase: Your Strength score increases by 2. Your Constitution score increases by 1.
• Type: Your type is Humanoid.
• Size: Your size is Medium.
• Speed: You have 30 foot land speed and swim speed equal to your land speed.
• Superior Darkvision: Within 120 feet, you can see in dim light as if it were bright light, and in darkness as if it were dim light. You can't discern color in darkness, only shades of gray.
• Amphibious: You can breathe air and water.
• Sunlight Sensitivity: You have disadvantage on attack rolls and Wisdom (Perception) checks that rely on sight when you, the target of the attack, or whatever you are trying to perceive is in direct sunlight.
• Otherworldly Perception: You can sense the presence of any creature within 30 feet of you that is invisible or on the Ethereal Plane. You can pinpoint such a creature that is moving.
• Slippery: You have advantage on ability checks and saving throws made to escape a grapple.
• Bite: Your bite is a natural weapon, which you can use to make unarmed strikes. If you hit with it, you deal piercing damage equal to 1d4 + your Strength modifier, instead of the bludgeoning damage normal for an unarmed strike.
• Languages: You know Common and Kuo-Toan.

Locathah 

I decided Locathah should be axolotl-folk.

The main deal with axolotls is that they are actually the tadpole form of a salamander which just never metamorphoses, just grows to an adult size instead. Thyroid hormone can be administered to induce metamorphosis into a salamander that hasn't been seen in the wild for millions of years. So, naturally, axolotl-folk can do that with magic.

Species Traits

As in Locathah Rising (Extra Life 2019), except:

• Replace the listed Limited Amphibiouisness trait with:
• Limited Amphibiousness. You can breathe both air and water. However, you cannot receive the benefits of a Short or Long rest unless you are fully submerged in water for the entire duration of your rest.

Feat: Metamorphosed

A Locathah shaman can cast the Ceremony spell on another Locathah to induce metamorphosis into a super-adult stage.

Prerequisites

• Locathah species.
• You must be under the effects of the Coming of Age Ceremony cast by a Locathah shaman at time of feat selection.

Effect

• You lose the effect of your Limited Amphibiousness trait, and are now fully amphibious -- you can breathe both air and water, and do not need to be submerged in water to gain the benefits of a Short or Long Rest.
• You have Resistance to Poison damage.
• Your skin turns a vivid color and begins to secrete a poisonous toxin. If you are hit by a Bite attack, the attacker takes 1d6 Poison damage. If you are subject to the Swallow Whole ability, the creature that swallows you takes xd6 poison damage on your turn every round you are swallowed, where x is equal to your Constitution modifier (minimum 1).
• You can harvest your own poison as per the rules in the Dungeon Master's Guide, except you do not need to be incapacitated or dead; you are not immune to your own poison, but you remain Resistant to its Poison damage. The harvested poison can be used as-is or refined:
• When used as-is, this is an Ingested poison that deals 2d6 Poison damage, Constitution save negates. A dose of this poison lasts for 24 hours before it becomes inert.
• Refining the poison follows the Crafting rules, but you have unlimited access to the ingredients. When refined, it becomes a dose of Pale Tincture poison, which lasts indefinitely until use.