I would say 8:15 would be a good time to request an ADR.
My best guestimate would be that it is a 20 minute show, give or take a couple of minutes. Allow another 20 minutes at least to get out of the theatre and out of the park.
The boats are slow and the lines become long, so your best bet is walking to the Boardwalk, a 15 minute walk. Just stay to your left...not all the way to the boat launch, just bare left as you exit and you will see the walkway.