You dont need a 100W diode. A standard silicon diode will have a Vf of ~0.7V. At 6A it will only dissipate roughly 4W. If you use a Schottky diode, Vf will be ~0.4V and you will only drop 2.5W. Schottky's generally switch faster than standard rectifier diodes.
In your case I'd pick a Schottky diode rated at around 10A, 25V (or higher) with a power rating of about 10W. That should give you plenty of safety margin. Digikey or Mouser will have a bewildering array of choices in that range.
I am assuming that by 'kickback diode' you mean you are putting a diode reverse biased across the pump, to protect the mosfet? Is this a PWM for speed control, or simple on/off?