I once dived with a guy who speared commercialy, and he got his kwota everyday, but did not ask what licences you need. They were four guys and was from the boat.
In my personal opinion, I think this is the most enviromental friendly way of catching fish, altough not the most economic or easy. Altough, if your working per kilo, the spearo might go for the bigger fish, while the guy with the line has to take whatever bites.