You honed senses gives you proficiency in perception. If already proficient it becomes expertise.

Your Dexterity increased by 2 and your Constitution by 1

Age. Selachifolk reach maturity at 20, and can live up to five centuries.

Selachifolk know that on the land surface dwellers are stronger. So they bend the rules, but try not to break them. Most are lawful evil.

Selachifolk are a bit shorter than average humans. They range from 4-5 feet tall. Your size is medium.

You are a creature made to live on both land and in the sea. You have both legs and a tail. Your base walking speed is 30, and you have swim speed of 40.

Your race never evolved the proper receptors, and because of this you are colorblind and can only see in shades of grey

You have lungs, but also gills, which are located under your collar. You can breathe air and water. 

Your senses are honed for hunting. You can see in dim light within 60 feet of you as if it were light, and darkness as if it were dim light.

You can speak, read, and write common and Aquan

You can trash your tail to propel yourself to absurd speeds. As a bonus action, your swimming speed is doubled until the end of your turn. You can't do so again until you finish a short or long rest.

Your sharp teeth are natural weapons, which you can use to make unarmed strikes. If you hit with it, you deal piercing damage equal to 1d8 + your Strength modifier. 

Due to being a race the lives at the surface of the water in tropical and subtropical waters, you are not adapted to colder temperatures. You have a vulnerability to cold damage.