There's a chemical called "Find A Leak" that you can use to exactly locate the leak. Turn the filter off and let the water settle. Then pour some in where you think the leak is and wait. The dye should collect at the leak point. I was looking for one leak last summer, and not only found it, but one below it I didn't know about!

If the pool is vinyl you can patch underwater with vinyl patches (I use a hunk of liner) and Boxer brand glue. If it's concrete there is underwater epoxy. If it's in the plumbing, you simply have to find it and fix the pipes.