I'd get a second estimate. Not knowing anything about the process to apply the Krystalkrete materials it sounds high.

I have a pool of similar size but not as deep (6' max end) and had it re-plastered with just plaster (no fancy stuff) for about $3500.00 4 years ago.

Living in Az I'm sure the competition keeps the price down a bit but $20K sounds like a LOT of money for that job.