Your dexterity score increases by 2 and you can increase two extra traits by 1.

Half-Merfolk share their mothers lifespan and reach adulthood in their late teens and live less than a century.

Half-Merfolk alignments depend on the upbringing of their human mother, though most tend towards the neutral stance of their fathers.

Half-Merfolk vary widely in height and build, from barely 5 feet to well over 6 feet tall. Regardless of your position in that range, your size is Medium.

Your base walking speed is 30 feet. You have an affinity for water and your base swimming speed is 40 feet

You can speak, read, and write Common and one extra language of your choice. Half-Merfolk typically follow their human ancestors ways and  learn the languages of other peoples they deal with, including obscure dialects. They are fond of sprinkling their speech with words borrowed from other tongues: Orc curses, Elvish musical expressions, Dwarvish military phrases, and so on.

Your eyes are evolved to see in dark murky waters, which of course helps above water as well. You have darkvision up to 60 feet and you can see in darkness as if the darkness were dim light,. However, you can't discern colour in darkness, only shades of grey.

You have taken on some traits of your merman father, including that of half developed gills. These allow you to breathe underwater but only for a short period of time. You can survive underwater for a period of hours equal to your constitution modifier plus one.

Half-merfolk are adept swimmers from birth and many find fighting in water as easy as standing on dry land. They do not have disadvantage attacks made against enemies when fighting underwater.