Predicted cell phone coverage maps for a number of common providers can been seen at the following address: http://www.comparecellular.com/coverage-maps/
If there is no cell phone coverage in the location you are canoeing it would be wise to have a satellite phone or personal emergency beacon which offers complete coverage to call for help or signal your location. Purchasing such a device may not be realistic for some people in which case simpler items such as having flares, flashlights, a mirror, the ability to make a fire, or even brightly coloured clothing/lifejacket can be used to signal rescuers. It is important to note that even if there is coverage predicted it would be wise to have some other backup form of communication or device to alert someone of your location.