The water level may vary considerably according to the weather, (rise in heavy rain, fall in very hot weather) with  a floating fountain this does not matter since the fountain is literally floating just under the water surface. (The nozzle is above the water surface)

 If you opt for a fixed static fountain ideally the pump should be mounted on a "plinth" (The higher the better) this keeps the pump off the pool floor, by doing this the pump can not suck in too much silt / algae from the pool floor, which may clog the pump or nozzle (unless you have a nozzle with a wide outlet)

 But by doing this if the water level drops too low the pump will then become exposed, which in turn may cause it to burn out if it is not pumping water. If the water level rises above the nozzle it will not give its original effect. Also the plinth must be heavy enough to keep still and secure.