check your Quebec regs, they are online. In BC we have spearfishing regs almost the same as regular fishing. One thing though, you must carefully read the regs for individual species, and more specifically, the method of taking. Some species are not allowed to spear, while being allowed to catch with hook and line. Read here:
It is a long read but well worth it. Spearfishing is detailed on page 15. No salmon and no sturgeon.
Also pay attention to maps, special areas, seasonal closures/opening and legal size of fish. There is a lot to read during long winter nights. There is also a bit of attention from locals to what we do, it being not very common to see swimmers in our frigid waters. Most people genuinely curios, but there is always someone zealous to suggest that you are overfishing or fishing in closed area, evern if you are not. So try staying close to daily catch limits. I had a few altercations, pretty damn close to starting a fist fight. Also pay attention and ask around for a beach access, this is not Greece or Mexico, waterfront properties often limit your water egress points.