Homebrew
Amphibious. The merfolk can breathe air and water.
Aquatic Movement. Merfolk are adapted to swimming, not walking. Any bonus to your walking speed instead gives you that bonus to your swim speed. Any feature that would give you a swim speed instead gives you that as a walk speed. Any feature that would give you a movement speed equal to your walking speed instead gives you that movement speed equal to your swim speed.
When moving on land you are considered prone. This form of prone doesn't slow your movement speed and only takes 5 feet of movement speed to end. This only applies if you didn't get a walk speed from any source.
Divers. Merfolk are adapted to the depths. You don't suffer any drawbacks from being at great depth in the water, be it pressure, temperature, salinity, etc..
Marine Recovery. If you complete a short rest while submerged in water during which you spend hitdie, you can regain 1d4 extra hit points. If you complete a long rest while submerged in water, the amount of hit die you recover increases by one. If you complete a long rest while not in water, the amount of hitdie you recover decreases by one, to a minimum of 1.
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