Aqualung Mini Zak is -as the name suggests- a small sized dive knife, which makes it great for a back up. It is available with either point tip (Mini Zak Alpha) or blunt tip (Mini Zak Beta).
We particularly like the Beta version as rarely you will have need for a pointed tip and should you miss the sheath, you will not accidentally cause damage to your gear with the tip. Also the blunt end functions as a handy replacement for a screwdriver.
This great little knife comes with a sheath and leg straps and two different mounting options on top of that! You can attach it to your bcd hose or your belt.