new user - leaky pool

18x36 inground, concrete floor, fiberglas sides. Losing 3-5 inches a day. Have plugged skimmer and return lines, still lose water so does not appear to be plumbing. No hydrostatic valve. Have started dye testing, so far all the obvious areas no leak. These are skimmer areas, return lines, existing cracks in floor, floor drains, light, and the seal between the floor and fiberglas panels. I have yet to test the seams between the panels. Once i find the leak, what is best underwater caulk to use on the leak?