Float is extremely important if you find yourself on your bike in deep water ;-)

Seriously, with a knee injury first and foremost go with a pedal that is comfortable. Float is important to some for different applications, but for commuting with a knee injury, I recommend the Crank Brothers line of pedals. Easy access to the clip in and depending on how you mount your cleats, the float varies. I don't need my shoes to be locked in one position - I can focus enough to keep my pedal stroke straight and do not require a no-float pedal to do it for me.