Breaker was in a sub panel located indoors near the pool.
The breaker was in the OFF position, not tripped.
When the reset was pushed ON the receptacle it would quickly trip.
The receptacle was on a dedicated circuit, no shared neutral.
Thinking that the breaker was defective the black wire to the breaker was disconnected.
The GFI receptacle was reset*, and then tripped !
*Older GFIs could be reset with the power OFF.