I think the water jousting is strictly a summer entertainment. We usually go off season so have not seen it but have seen pictures and talked to people who have enjoyed it. March can be a bit chilly, even in the south, so water sports may not be a great idea. There is always sailing, boating, sail-surfing and using wet suits to consider. The French are seriously into sport so I"m sure you'll find something appealing to teenage boys. They can rent bicycles or Vespas and tool around the countryside.