We bought a house with a fancy walk-in shower made all of travertine tile, including a built-in bench (this work was done just before we bought the house). About a year later, we noticed water in the basement below the shower. We figured out that there were two problems: 1) the travetine bench was separating apart, letting water 'inside' the bench, and 2) somehow this water was not getting caught in the pan (we suspect that whoever put in the shower didn't install the pan correctly).

Long story short, we didn't want to spend a fortune to rip out the whole shower. We had a new bench made of Corian so that it would be one-piece and couldn't let water beneath/through it. This solved the problem except for one little thing. Where the top of the bench meets the wall (where Corain meets Travertine) it was caulked, and after another year or so that caulk wasn't adhering anymore, and now there was a seam for water to get behind the bench again (and into the basement).

I have made a repair to this about every 9-12 months since, trying a different product each time. Everytime I make the repair, the water in the basement stops immediately (so I'm dead sure this is where the water is coming from), but nothing sticks to the two surfaces for very long. I suspect that expansion and contraction of the two surfaces doesn't help. I've tried very hard things (epoxy, superglue) and flexible things (caulk, marine glue, silicone).

Any ideas what will bond to both of these surfaces and stick for a good long time? Any other ideas how I can permanently fill the sub-millimeter gap that is letting water through?