It it safe to have a 15 amp GFCI outlet on a 50 amp GFCI breaker?
This is trouble, but easily solved.  Simply swap out the 50 ampere GFCI breaker, for a 15 ampere GFCI breaker. 

You'll possibly have to use pigtails to connect to the breaker, as it may not accept the size wire used for the existing circuit.



As "subpanels" seem to be quite popular around here, I'm surprised it hasn't been suggested yet.  You could always install a second panel where the tub used to be, then install a 15 or 20 ampere breaker to feed the receptacle.