Activity is always good. And certainly you are going to be working many muscles to keep yourself upright and balanced.
But I would hesitate to consider these cardio, as they aren't going to working your heart and lungs hard on a sustained basis in the same way that more focussed exercise would.
And I'm not sure that a Heart Rate Monitor is going to produce a reliable estimate of calories burned either, as your heart rate is like to be elevated due to the "adrenaline rush" rather than how hard your body is working.
So go and have some great summer fun, but don't stop doing more conventional exercise as well. In fact, some increased strength and fitness may help improve your wakewboarding, etc.
I would say they wouldn't be cardio. For those activities something is pulling you along and you are balancing. They would be awesome for stability (you would need a strong core), but since you are just holding yourself steady I wouldn't call it cardio. Muscles would get a good workout from holding you up, though!
While I have never done any of those particular water sports, they sure sound like a lot of exercise to me, and a heck of a lot of fun! Swimming and water skiing (two feet) sound boring compared to your list! I want to go with you!!! But I live in Washington State. Darn....
They are cardiovascular (cardio) if they keep your heart rate up for 20-30 minutes.
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