Hi Donald, 

Apologies for the long wait time in responding. We recommend using 2-4 tablespoons of salt per gallon of water for cooking crabs before processing, depending on your taste preference. Plain table salt works just fine for salting the water you'll boil the crabs in. Note that if you use kosher salt, the grains are much fatter and you get less salt per tablespoon; to compensate, you can add 25% more kosher salt (so 2.5 to 5 tablespoons per gallon of water). 

If you want to cook them in water that is truly as salty as the ocean, you'd need to increase your ration to about 8 or 8.5 tablespoons of salt per gallon. You might not like how salty the end product tastes, which is why we recommend a lower ratio. 

If you're looking for further info on processing crabs, the two documents I'm attaching give instructions for canning and for freezing. 

Hope this is helpful!

Jared