Jippie's answer (put as a comment) is largely correct.
Current = Power/Volts = 25/12 ~= 2A.
Substantially more current flows at startup and the diode must handle this.
Diode D2 is not needed to achieve your functionality (just replace it with a wire, as he says). Using D2 may be useful to provide both motors with the same voltage (as both then have 12V - 1 x diode_drop but you will probably not notice the voltage drop across the motor when the pump is pumping.
Many diodes will meet your need. Using Digikey's parametric selection guide I specified a diode of at least 5A rating and 40V reverse rating (safety margins never hurt) and asked for the cheapest silicon diode in a leaded package. Surprisingly it was [this 600V 5A BY500] (http://www.vishay.com/docs/88544/by500.pdf)
This is far more capable than needed but cheapest is cheapest :-). About 53 cents/1 at Digikey. (I tried specifying a >= 2.5A and >=20V part and the above was STILL cheapest :-) ). You can actually get a Schottky diode for less, and these are in fact superior technically BUT are more easily killed when playing so specifying a standard Silicon diode is safer here.
Be sure to be sure that the switch will handle the 12V inductive load of the motor.