As others have said, it will be tight.
RoL at 6:15 should be done about 6:30. Depends how long it takes you to get out of the theater and to the front gate, could be 30 minutes or more.
After RoL, there is a mass exudus out of the park. A big mob heading for the gates, trams to cars and a lot of cars all trying to leave at the same time. Anyone guess how long that takes. You could time it right and not take long at all. Driving to Boardwalk 20+ minutes depending on traffic. Parking and walking to Flying Fish.
It could work. I would try moving the reservation back to 8:30 and you should have no problems and not have to stress about it.