On the GFI outlet you have a Line side and a Load side.
The line side will supply the GFI outlet's with power.
The load side will simply supply power to any other devices you add on for instance another outlet or multiple outlets. Now your GFI being first inline will protect the added devices.
So in a since your switch will be protected in two ways, one if your pump fails it will trip the GFI first and protect your line voltage. Second what ever outlet you plug your extension cord into will ultimately be protected by the main breaker at your homes service panel. You may want to consider plugging into a GFI outlet to protect line voltage to your remote GFI/switch .
Attached is a quick hand sketched schematic. Using the switch as the on/off to the GFI line voltage and ultimately turning on/off your pump.
OK so for what ever reason I could not upload my sketch. PM me with email address and I can send it to you.
For now I will quickly explain.
Hook your wires up to the Line side only.
Your white wire will go to the neutral side terminal of the GFI.
The green wire obviously will go to the ground terminal on both the switch and GFI.
The black wire will go to one side of your switch. Then you will need a second piece of wire to connect to the other side of the switch and then connect to the HOT side of your GFI.
The switch will now interrupt the supply voltage to the GFI turning the outlets/pump ON and OFF.
Hope this helps.