cable will save money and give you a good base, get the basics down then take it to the boat, you won't have to pay near as much gas then... if you can ride cable boat will be easy
I would go behind a boat. I enjoy being out in the boat chilling with friends almost as much as riding. Where do you live? If you are near MN or WI, you are more than welcome anytime to ride behind my MC (when our lakes thaw out).