Can you not derive it directly from the conservation of energy? For every unit volume of water leaving the tank, the water level in the tank drops. Thus the tank has lost some gravitational potential energy, and produced some kinetic energy, and the tank might be thought of as a device that converts energy in this way.
Equating the energy input with the energy output should give you the desired relation.
This is not entirely surprising because Bernoulli's equation is based on energy conservation.