Casa Bigote is the restaurant that put the Bajo de Guía seafood strip on the map. It's been operating since the 1960s, serving the same formula of impeccably fresh fish and shellfish with manzanilla from across the road. The langostinos (large prawns) are the thing to order — they come from the estuary just outside the window, grilled whole with sea salt.
The dining room is plain and a little worn around the edges, which is entirely appropriate. There's a terrace for warmer months. Booking is essential at weekends and in summer. If you can't get a table, the tapas bar next door serves the same produce at a lower price with less ceremony — stand at the bar with a glass of cold manzanilla and a racion of gambas and you'll understand why people come to Sanlúcar.