This is based on the standard spider 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: 15 ft. climb 15 ft.
- Walk on Water: you can use you normal walking speed on water. The water tension is enough that you do not since in the water as you walk across its surface. You are very attractive to fish while you do this.
- Natural Armor: You have rigid chitinous armor. When you aren’t wearing armor, your AC is 13 + your Constitution modifier. You can use your natural armor to determine your AC if the armor you wear would leave you with a lower AC. A shield’s benefits apply as normal while you use your natural armor.
- First Level Diverse Features: Grounded, Tough Hide
- Recommended Diverse Features for 5th and 10th level: Natural Weapon, Blend In, Bony Exterior, Canopy Cover, Climber, Digger, Durable, Exceptional Jumper, Forager, Giant Leap, Improved Attack, Leap, Pack Tactics, Tough Exterior