The water won't drain back down the well, you'll have a check valve at the pump itself at the bottom of the well. If that well is 600 feet deep the pressure down there is pretty amazing, like 240 psi plus whatever pressure is present at the surface.
The check valve at the pump is another reason that it is so hard to pull the pump back up for service, you're talking about lifting a 1-1.5" diameter water column that could be 600 feet long plus the weight of the pipe, wire, and pump. When you lower the pump it is much easier without the water weight.
OK, a water column 1.5" across and 600' long is 55 gallons of water and weighs 412 lbs.