Catching crab is a blast, it is also my favorite food. Be sure to know the local catch laws and buy a license. Canada has a different size requirement and measuring method than in Washington or Oregon. Most regulations require you to keep the Carapace (back shell) on the crab until cooked & consumed. There are catch and possession limits.
A long time ago a kind boater showed me this quick and easy method for cleaning crab. It takes less than a minute. I showed my dentist this on the back of his sailboat, I would of thought after 4-years of Dental School he would have been less squeamish...
Hold the crab safely, on its back so it passes out. Keep your thumb out of the claws reach like shown
Sneek your hands up each side and hold the claws, now you have him totally controlled.
Split the crab in two, quickly and firmly. Cleats on the dock are good for this.
Twist off the Carapace, it will be pretty loose.