Water/antifreeze radiator cooled work shop
There are air conditioners using sea water, rather than a heat pump, but that works because ocean water is usually well below 10° C (50° F). Using a canal would be less corrosive, but you'd need a heat-pump to get useful cooling. Some examples of air conditioners using sea water to cool the condenser are at Flagship Marine.

That said, you'd need to check local regulations for use of the canal water.

As has been mentioned, though, making the workshop fairly air-tight and well insulated is necessary before you install air conditioning, or you're pouring money for the air conditioner and the electricity to operate it out the window.

Solar panels on the hot tin roof would help to shade the building and run the AC, too.