In ground pool has pool drain cover plate trying to poke through the liner

I had my liner replaced two years ago, the installer suggested removing the bottom drain for child safety. As I have a young child I took the advice and he proceeded to close off the bottom drain. He removed it and placed a large metal sheet over the hole. He taped down the edges with duct tape. Almost immediately after filling with water the metal sheet has been trying to pull up the edges are bent up and pushing the liner upward. I think its just a matter of time before they cut the liner. I dont see how one can bend the metal back down the liner has gripped the metal so hard im afraid of trying to pound the metal for fear of cutting the liner. So I guess im going to have to patch the hole, when it comes. How is that done? Do you have to drain the water?