And more on the epoxy front... I have successfully used QuikSteel epoxy putty (found in the automotive section of Walmart and other places). It bonds to wet or dry surfaces and will even set up underwater. I used it to fix a sizable crack (1/2" wide and 3 feet long) underwater. It's a little messy. You kneed the dark grey putty together until it's all the same color and a little warm. Then you cram it into the crack. As with all repairs, be sure the crack is as clear as possible. Wirebrush it to rough it up, even undercut the crack a little if you can. If you can get it to bond well, you should be all set. A section of my crack didn't bond well (algae in it, I think) and the putty just peeled off. But the part that did stick, that was the most important, stuck very well! I've also used it to fix some rust spots around the windshield of my old convertible. BasilB

Uh oh... Thing is, grout isn't waterproof: it's there to fill up the space with something that wears well, it doesn't keep water from migrating. Slows it a little, doesn't stop it. So the grout cracks are a problem, but they are the least of your problems. You need to evaluate how this shower was constructed, and where the system is breaking down. The experts here [I'm not one] may be able to give you some advice as to where to start, but if I were you, I wouldn't use this shower again till a plumber takes a look. And if he tells you the grout is the problem, find a new plumber. Sorry.

A Likely Solution? I believe I have found and stopped the leak. At least for now, and seemingly for the most part. I'm posting this in thanks for those who contributed above, and hopefully to help others who may encounter a problem like mine in the future! I was loosing 4" overnight and now, it's maybe 1/4" or less overnight. (Temps here are 40-50 so evap should be at a minimum). I realize I still may have some small leaks. I spoke at length with a guy who plasters new pools professionally, asking about those hairline cracks. He said that since they were above the winterized water level, he suspected that they were due to drying out of the plaster with the water level lowered. He finds that the hairline cracks like mine usually disappear once the water level is raised, due to adding moisture back to the plaster. He said that in 20 years of plastering pools, he had only found one pool materially leaking through this kind of hairline crack. So just to be cautious, I did apply a skim coat of silicone to them before refilling, though they were not my main suspects! I got on a mechanic's creeper and spent several hours examining the tile line around this 40' pool. I found about 8-10 openings in the grout between those tiles at various places from 1/4" up to 2".I cut out the bad grout and replaced it with silicone for a temporary repair. I know that matching grout might have been better - I just didn't have any available on short notice. I also spent a fair amount of time in and around the pool side openings to the skimmers. There was some obvious missing grout there, and I did carefully seal the vertical and horizontal joints between the skimmer box and the plaster. (On my pool, that vertical joint was up inside the skimmer, maybe 4" from the face of the pool wall) I let that cure about 8 hours and filled the pool. So far it's 95% better than before! Now that I can finally keep the water level up to the skimmers, I can finally run the pump . Once we get a clear pool, I will dye test everything around the tile line, the skimmer box, and those pesky hairline cracks, just to make sure. Interestingly, in posting this on this and a few other forums, I don't recall anyone suggesting a look at the tile line. Thankfully, numerous folks did suggest having a look at the interface between the skimmers and the structure. It's a great relief so far, though I know I will have to keep an eye on things, and maybe later cut out the silicone and replace it with grout!

Underground water may present some issues as the water is capable of moving the base material used, be it sand or small gravel though gravel would be somewhat surprising due to it's weight. The real questions that beg to be answered from what I am hearing are: What was the base material? Where did the base material go that was there? When was the high water table realized? Was anything done to redirect the water away? Was the material under the added sand or gravel a suitable base material? Clays should not have been left. They swell and shrink, creating voids. How did the pool level drop? Turning on the sump should only be done after the other sources are shut and the discharge sent to waste and at least 75 feet or more away and down hill or to a storm sewer that carries it away. If the water under wasn't removed but the water in the pool was, it could be partially your fault. I hate to say this but you might let a lawyer not affiliated with the installer know what is going on an initial consultation basis and if the installer doesn't bring proper satisfaction... God, I hate saying that. Scott