Sailing and flying?
At the end of the day you could buy a sailing boat and just go off in it....... foolish?
There are qualifications relating to size and complicity of the boat, be they you just want to mess around on a small inland lake, or if you want race. Different levels, different qualifications and different costs. Also, instructors have to re-new there qualifications periodically at a cost each time.
Its roughly the same for power boats too, you could just buy one and go - but most people do cources, from just how to drive one to how to do "resuce boat" or race. Again, different levels, different qualifiactions, different costs.
FW