Why does he have to use #12 wire? Why couldn't it be a 15A circuit?
He HAS to use #12 because the code specifically requires it.
He HAS to use a 20A circuit because this is one of the few residential circuits considered "continuous" in the eyes of the code.
The portion of the wiring that is inside the house can be regular NM cable, but once the circuit penetrates the outside wall of the house it MUST be insulated conductors in conduit.
The cord for the pump cannot be more than 3' long, and must be #12 minimum.
If the receptacle for the motor will be near the pool it cannot be less than 5' from the pool wall.
If it is between 5' and 10' from the pool wall then it MUST be a twist-lock receptacle.
Then there is pool bonding, which is different from grounding.
I could go on....
Also, there are a few changes in these codes recently. We would need to know what code cycle the OP is working under to be accurate.
There are SO many codes involved with wiring pool equipment that folks ignore or don't bother following. They are there for a reason.
This is one of the key reasons you see so many free used pools. Folks put up a pool and never bother to do it right. Then when they go to sell they are forced to either do it right or get rid of the pool.