I just re-read your post and thought of something I found on an old pool once. You said there was no main drain, but there was something that looked like one. Most gunnite pools have an opening in the deep end with a cover. The term main drain refers to a pipe that comes from the deep end goes to the pump. This allows water from the bottom to circulate through the filter. The pool should have one of these. Sometimes the main drain piping will develope a leak and in the interest of economy the owner will plug that off and rely on the skimmers. Additionally gunnite pools many times will have another plug inside the grated enclosure. This plug is the hydraulic relief. Many people think this is a drain plug. It is actually there to be opened when the pool is empty in order to relieve water pressure outside the pool and prevent the pool from floating. I have seen several arrangements where either plug (hydraulic relief or plugged main drain) have been rubber or have rubber components that failed from age. Dive down and look for yourself and see what is there.