Assuming you have a DC motor...
Most standard DC motors can run with a variety of voltages... lower voltages mean they run slower, higher voltages mean they run faster.
Certainly they should be able to tolerate 20% or so higher or lower voltages. But you can confirm with your manufacturer. Also note, the more amps you draw, the lower the voltage at the panel will be, and it can fall off quickly near the peak power point on the panel (under the current conditions).
What about startup power? You might be able to add some capacitors to aid with startup if necessary, but if your panels produce marginal voltage (including morning and evening times), then you might not be able to start the pump. And one of the hardest things on electric motors is being stalled.
What about an old fashioned windmill for pumping water?
I was also reading about enphase micro-inverters. 240V AC, 175W with MPPT, and can daisy chain together if you need more power. The basic unit is about $200 each.
