Billed-Bladet has a short article about the Crown Prince couple's family holiday in Sardinia:
Billed-Bladet - Frederik og Marys fede familieferie på Sardinien
They are staying on a yacht. Christian went for a ride on the jet-ski with his parents, and loved the speed over the water.
MSN.dk also has some photos of the family on the jet-ski (minus Isabella-she was too young to ride on it):
http://msndk.starlounge.com/index.cfm?objectid=56676