I live in Toronto, Canada. I buy my mussels from Diana's Seafood or Loblaws.
I wonder if the quote beneath from Mussel Myths & FAQs - The Cornish Mussel Shack holds true for Canada too?
Do I need to soak my mussels in water and oatmeal?
No. This used to be done to help purge the mussel of any grit. All commercially sold mussels legally have to be purged and purified in UV filtered water for 42 hours. During this time most of the grit and sediment is released from the mussel.
Do I need to soak the mussels and throw away the ones that float?
Salt water mussels should never be soaked in fresh water as this will kill them, and as explained in the above answer there is no longer any need to soak mussels. This was done in the past to identify dead mussels. People thought that a floating mussel was a dead mussel, which is also is not true. A live mussel can often trap an air bubble when it snaps shut, and this is why some mussels float. To identify dead mussels see our page on preparing mussels safely.
How to Clean and Debeard Mussels | Serious Eats says something similar, but nothing about the law.
As an added bonus, most farm-raised mussels are grown on vertical rope farms, which means that they come to market quite clean—wild mussels can house a good amount of debris from the seabed or rock walls they grew on. On top of that, farm-raised mussels are held in tanks prior to packaging and shipping, which means that the purging step—soaking the mussels in clean water until they spit out impurities—has already been done for you.