canal=* is infrequently used to further describe the type of waterway=canal. Most of the current values have not been defined.
Other more frequently-used keys with a similar purpose include usage=* and service=*
canal=qanat and canal=khettara - for a qanat, also known as a faraj, khettara, kariz or kanez - a gently sloping underground channel used to transport water downhill from an aquifer or water well to the surface, found in the Middle East and surrounding arid areas near hills or mountains.
canal=leat - a leat, an artificial watercourse dug into the ground, especially one supplying water to a watermill or its mill pond, usually found in the south and west of England and in Wales.
canal=irrigation - a canal used for irrigation. Consider instead the more common tags usage=irrigation and service=irrigation.
canal=disused - for a canal that is not currently used, but could be repaired. Consider instead disused:canal=* or disused=yes
canal=abandoned - for a canal that is no longer functional. Consider instead abandoned:canal=* and abandoned=yes.