When mine was dug the bottom of the deep end was soft and like quick sand, but it was a good time of year and we were lucky the water table was not higher. We got the rock in and it was pretty solid. When my plaster was put in two days ago, we just got hit with Andrea and the water table was up 2' in the pool. They hooked up a pump to the drain to pump water from under the pool for 30 hours while they plastered and acid washed. When they removed the pipe I expected to see water rushing in, but it did not. Over the 10 minutes it took them to get the drain plugs in, it rose a a few inches slowly.
What I was getting at with the long story, is that a well point, as suggested above, with a pump running for 12+ hours should drop the water table in that area low enough for you to dig, fill with stone and compact the area. You will probably need the same well point or deep end drain when you shoot the gunite and plaster. My drains has a 2' 1-1/2" PVC tube I made with holes drilled in it, in the bottom drain fitting. It was bedded in gravel. That allowed it to really get good flow.