Desalination does not use heat anymore, it's all membrane based.

Also, you'd need enough heat to boil water, but the waste heat, by definition, is lower than the boiling point of water (otherwise the turbine doesn't work).

Desalination can be done through osmosis, which is membrane based. It can also be done through evaporation, which is much less efficient. But if you have a free source of heat, that inefficiency is a non issue.

The waste heat from a steam turbine is higher than boiling. Otherwise the turbine wouldn't work. The boiler turns water into steam, which produces pressure to drive a turbine. The condenser takes the low pressure steam and condenses it back to water.

That would only help near the coasts. Most of Canada's population is in southern Ontario and Quebec where there's no saltwater.