Zener Diode: Easing the load on a regulator
In principle this will work- the zener would have to dissipate more than 3W, so it will have to be something like a 5W zener- not cheap. The heat has to go somewhere in a linear regulator, adding the zener will move part of it to the diode.  

The resistor will work if the output current is a constant 1A current, however motors, especially those used in applications such as yours, tend to draw much higher current during starting so it may end up stalling your pump and burning up the resistor. 

At this level, I strongly suggest you use a switching regulator (buck regulator), such as those based on the LM2596. It will run cool. Inexpensive modules are available from China which may or may not use genuine LM2596s, but seem to work well enough.