I agree with @janbb that in March Florida is the way to go. The weather is usually ideal in March. Very little rain, springlike temperatures, but there is no guarantee of course. In March you will be better off staying towards the very southern part of the US or you risk very cold weather.
I found this site and this site for places to canoe and kayak in FL. Probably FL tourism could help you too. Each state would have similar information and their are sites for top ten places to canoe in the country, but some of those could still be in the tundra during March.
Depending where you are coming from FL might be very far, so do consider southern states more west also. FYI in some parts of FL the week before Easter and the week after are the most expensive and most crowded with tourists.