As you can see, spearguns can cost you a pretty penny. Given the fact that you want to protect your investment, the worst thing that could happen to you is if you lost the gun while underwater. Remember, while you are using it, your hand-to-eye coordination is not going to be the same and the gear you are wearing can also cause issues. So, instead of merely relying on your grip, you may want to secure the gun to prevent accidental dropping.
One of the ways you can prevent this is to secure the gun to a floating line using a shark clip. This way, if you happen to drop the gun, it will be very easy to recover in poor visibility conditions.