I think not. NM is not one of the approved wiring methods in 680 part II. I also could have problems with the ampacity, based on the nameplate rating of the spa and mfg instructions. 8ga is only good for 40a in the 60c column.
Shorties right read your Manufacture's manual most spa manufacture's will put min. wire size and I've found more times tahn not it is a #6 copper, but check your manual. If customer doesn' know call dealer they bought it from.
A spa is a combination load with a significant part that is a "toaster wire" heater. The answer is in the manufacturer's instructions unless you want to start reading nameplate ratings and calculating. You are still stuck in the 60c column with Romex.