A hydro is a shorter way of referring to a hydroplane which, to the uninformed, is a boat that actually flies over the water using ground effects to support it while at racing speeds. The boats are, for lack of a better way to put it, an airfoil with stabilizer floats along both sides. I race 1/8 scale and smaller classes of hydros in the Pacific Northwest, hence the name. As far as the jet rallies, never been to one though, years ago I did see some flying from the Coupeville runway complex the Navy maintains for carrier landing training.
This will probably have some asking why I would be in this thread. I'm building several boats AND AIRCRAFT at the moment and want to be able to fly the planes when done. The people that are recklessly operating(I can't call it flying since electronics are actually doing that) these quads are endangering my right to fly my planes when completed, thus making the possibility that my models will be relegated to shelf queens a real issue