This is based on the standard sea horse Familiar from DND 5e. The standard stat block would be the same for the familiar itself, below are the quick construction guide for the bat.
Species attributes:
- Speed: 10 ft., swim 30 ft.
- Aquatic: You can only breathe underwater. You can hold your breath and survive outside the water a number of minutes equal to six times your Constitution score.
- Skitter: As a bonus action, you can take the Dash action.
- First Level Diverse Features: Bony Exterior, Fleet Feet
- Recommended Diverse Features for 5th and 10th level: Natural Weapon, Directional Camouflage, Durable, Hard Biter, Hunter, Sensitive Strike, Storage Mouth, Swimmer.