"People eat everywhere" is a useful phrase to keep in mind when provisioning for a voyage. Wherever there are people you can get food. The conditions are that you may not like what they eat and where there are no people there are no groceries.
Mexican supermarkets are comparable to those in Canada and the US. They have similar kinds of goods, often the same brands, at about 25% lower prices. Mexican towns will also have a "farmers" market which often has the best selection of produce.
Produce is different. For the most part produce on the Baja is terrible, even in the big groceries in the cities. Basics like onions, potatoes, and garlic are usually good, the more perishable vegetables are hit and miss. Luckily, limes, oranges, and avocados are good, cheap, and plentiful.
On the western side of the Baja there are no supermarkets between Ensenada and Cabo San Lucas. There are two Soriana's (brand) stores in downtown Ensenada, the one five blocks further from the shore has a better selection. I recommend having about a month's worth of supplies when you leave Ensenada. You can get bread and milk and other basics in Turtle Bay, but otherwise you're on your own until you reach Cabo.
On the Sea of Cortez side things are a little better, but most of the towns don't have good anchorages. And the good anchorages don't have good groceries (if any). So stocking up in La Paz is a good idea. We liked CCC which targets the gringo market so they have have a good selection of things that aren't available elsewhere, for example pickled Okra and Stoned Wheat Thins. Of course, this means they are more expensive, if still cheap compared to Canada.
On the mainland the cities have excellent supermarkets. However they have very limited selection of canned vegetables, for example no canned tomatoes or plain beans.
Tortillas are a nice addition to our diet. The flour tortillas here are different from the ones you get in US/Canada. They are softer and sweeter, more like crepes than wraps. And they are cheap and available almost everywhere.
Liquor prices in Mexico are all over the map. Rum and Brandy are cheaper than in the US, but gin or Irish Cream are about the same in Mexico as in Canada. Local beer is cheap but selection is poor. If you like dark beer stock up before you arrive. Wine prices are much higher and selection much worse than in the US. If you drink gin, Irish Cream, or wine stock up before you arrive.