How Scuba Certification is actually enforced 

Death, death is usually a consequence of diving without a cert. (Most places don't want to deal with your death so they won't let you do it)...

Considering the cannon idea now….

I always heard that the main reason. Is so you can get nitrogen mixed diving gas. Otherwise, if you compress your own tank; their is a nitrogen narcosis depth limit. So certified divers can go deeper.

how to build cannon