Have you considered dropping a vinyl liner in it? I think you could get a good concrete coating, painted on,, then tile over that and use a polymer 'water proof' grout. You'd have to either do some research on the best combination of 'coating/paint', tile,, mortar for the tile and grout for between the tiles OR call the pros. I have a concrete pool with a vermucilute floor, I dropped a liner in it = but originally intended to tile the entire inside,, but was too concernted about the concrete cracking over time and therefore cracking the tile,, and having major leaks.