This is based on the standard octopus 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: 5 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.
- Jet: You can use the Dash action as a bonus action on your turn. This ability recharges after a short or long rest.
- First Level Diverse Features: Grappler, Skitter
- Recommended Diverse Features for 5th and 10th level: Natural Weapon, Bony Exterior, Chameleon Skin, Chemical Sensitivity, Darkvision, Hard Biter, Ink Cloud, Skitter, Stalker, Strangle, Swimmer