You messed up the metaphor
The speed of the flow of water is the voltage
The amount of water in the river is the amperage
Rocks are resistance
The motor in your AEG is like a water mill, takes a specific amount of v and A to turn. Not enough water in the river means there isn't enough torque to move the wheel.
and read this if you're considering LiPo; http://airsoftcanada.com/showthread.php?t=99899