You must use conduit and individual conductors for the outside portion of the wiring. This part must have an insulated ground. You can come into the back of the weatherproof box with the NM cable where you change over to PVC conduit.
The inside portion can be NM cable and the bare #10 ground is fine.
Keep in mind you can only have 6' of flexible. Use some rigid PVC to get closer to the tub then change over to sealtite.