Ground Water while replacing the Liner

[drdingo]

We are replacing the liner. Everything looked great till we go to the depend and we have groundwater leaking in. There is a crack along the back floor of the deep end that is allowing water in. There weren't any issues with the liner floating or anything like that but the leak is fast enough that we can't plug it with hydraulic cement. We have been pumping the water out periodically throughout the day but we would really like to speed up the process of getting rid of the water.

My question is how do we get rid of the groundwater to make repairs? My thought is the main drain has a plug on the bottom that has a square receptacle- can we remove that to allow the groundwater in so we can pump it out? I don't have the tool on hand to remove it but I did have a bar that fit it for the most part and gave it a good effort but the plug didn't move. I didn't try harder because I didn't want to break anything:

 

[drdingo]

Another question - The plug appears to be the backside of a normal Skimmer plug as if they plugged it from the other side before installing the drain. If that is the case is there a reason I can't drill it/cut it out to get the groundwater up and then replace it from this side once the water is drained?