I've had good success cleaning my water line tiles with suncoast "tile and liner cleaner". It's basically an acidic gel that can be thickened by adding a little muriatic acid. A little bit of that with a scrub pad does a fine job. I'm sure there's other brands of similar product.
There's another old thread on this topic here: http://www.poolforum.com/pf2/showthr...-tile-cleaning
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