Swimming Speed

Your base swimming speed is 40ft. Once per long rest, while underwater, a mermaid can activate their speed boost and travel up to 3000ft per round (500km per hour) for up to 2 hours.

Shapechangers

If you come into contact with water for 10 seconds, you transform into your mermaid form. You change back when you're completely dry. Your posessions and clothing transform with your body.

Underwater Breathing

Merpeople can stay submerged for long periods of time but they still need to breathe air. Merpeople can hold their breath for at least 12 hours before needing to resurface but with more practice merpeople can stay under for much longer. When mermaids remain breathless, they do not die, unlike humans. They "sleep" underwater, and just awaken when they breathe air again. They can only hold their breath in mer-form.

Tail Attack

The mermaid can make an unarmed attack with its tail whilst underwater. This attack creates bludgeoning damage of 2d4 plus your strength modifier.

Darkvision

Accustomed to life underwater, you have superior vision in dark and dim conditions. You can see in dim light within 60 feet of you 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 grey.

Speak with Sea Creatures

Through sounds and gestures, you can communicate simple ideas with creatures with a swimming speed.

Durability

Merpeople can swim for days without fatigue.

Cultural Abilities

Due to your upbringing, you have proficiency in all Nature, Survival and Investigation checks relating to water.

Full Moon Madness

If a mermaid views a full moon, they will have to perform a Wisdom save, or roll on the Full Moon Madness table.