We ate at Chez Black's in 2006 and it was delicious. Maybe not the best option but it's walkable to the beach and the hotels near the beach, and even our travel agent (who knows Positano well) from that trip highly recommended it.

http://www.chezblack.it/

For more "authentic" meals, take a bus or taxi to Ravello or other nearby places. Or just ask your hotelier for other suggestions...

We haven't been back to Positano only because we have chosen to explore other parts of Italia instead since... but yes, we do want to return to Positano / Capri and a lot of other parts of Amalfi Coast.