You can use an air mask and portable air compressor like the ones pool companies use and leak detection companies have. You can patch a hole that large. If you have a void area under the liner from the water leaking out you can fill this void with sand before you patch (make sure there is not any sand on the liner bottom when you patch). When you put the glue on the patch fold in half then go to the bottom unfold the patch and apply. Almost forgot, be sure you use a weight belt so you can stay on the bottom. Hope this helps