My question is in the title. Does it mean, if the canal is for boats or ships (transportation), it is called "运河"? If the canal is for watering the land, is it called "渠"? I am confused.
You got the point.
"运河" must refer the human-made channels in a large scale, for water conveyance/transportion. That's what "运" emphasize. Usually, it connects big lake/river/ocean or through the peninsula or isthmus.
In the meantime, "渠" just refer all human-made channels. "渠" is more like a superset of "运河";
But why we don't just call all channel to 渠? Same reason like the water is a superset of the ocean, but you would never call "the Pacific Ocean" to "the Pacific Water".
Now you can tell the difference between "运河" and "渠".