I agree with the personal locator beacon and VHF comments. Either (or both) would be good. Spot is probably the best known of the PLBs. You wouldn't need something as formal as the EPIRBS that larger boats often use. These devices are great for getting word that you are in trouble out and where you are at, but they are one way (they only say someone is in trouble and their location - not type of trouble nor can the rescuers communicate back).
If you are in an area with coast guard base and/or harbor patrols and/or lots of other boats, then a VHF would be good. These are line of sight radios, so only transmit over a few miles. In heavy traffic areas, Coast Guard may have repeaters, which extend the range a lot. If you are paddling away from civilization, then they aren't that useful. But in urban areas,they are good.
In urban areas, you can use your cell phone for emergency communication. But you need to have a way to keep it dry while you use it. There are special made dry bags for these.
In all cases, you likely want the communication method attached to you, not the boat. If you get separated from your boat, you want to still have access to this.