You are used to dark areas, and your eyes have adjusted to such. You can see in dim light within 60 feet of you as if it were bright light, and darkness as if it were dim light. You can't discern color in darkness, only shades of grey.

Your Dexterity score increases by 2.

Black rats can store up to 1 cubic foot of items in total in their cheek pouches, and they can transfer a single object between hand and cheek as a reaction. As a bonus action, a black rat can disgorge the entire contents of his pouch onto the ground.

You have advantage on Acrobatics checks and Dexterity saving throws to maintain balance.

You have advantage on Perception checks based on smell.

Ratfolk grow up learning how to survive in tough conditions. They are taught how to forage for food and move around their surroundings at a very young age. Because of this, they are proficient in Survival

You suffer no penalties due to squeezing into a smaller space.

Ratfolk have claws that can be used to deal slashing damage as an unarmed attack, doing damage equal to 1d4+DEX modifier. Classed as a light, held, finesse, melee weapon.

Ratfolk can choose to make a bite attack as an unarmed attack, dealing piercing damage. This deals damage equal to 1d6+STR modifier.

Due to the nature of the harsh conditions Ratfolk have endured over many generations, you have resistance to poison damage. Additionally, you have advantage on Constitution saving throws against disease and infection. 

Through sounds and gestures, you can communicate simple ideas with Small or Smaller beasts. Black Water Rats have a natural affinity for small animals, both from their original ancestors and their time amongst them.

Because of their life underground, Black Water Rats have superior vision in dark and dim conditions. They can see in dim light within 60 feet of them as if it were bright light, and in darkness as if it were dim light.