Fresh fish is hard to find in Sidari, although Dorothes Corner does have fresh fish (the owner lives in Corfu Town and he brings fresh fish from the market).
I can highly recommend taking the 5 minute drive from Sidari to Astrakeri Beach where you will find one and only one Taverna called the 3 brothers. It is a true Corfiot fish Taverna, 2 of the brothers are twins and they are great guys. You can look and choose your fish from the ice fridge, everything is fresh. The speciality is Bourdeto, a Corfiot dish of dogfish in a spicy sauce.
I tend to go in the evenings to the beach with my little girl and then we walk over to the Taverna for dinner at sundown. Mainly Greek clientele and if you go on Sunday lunchtime, get there early. The local choir eat on Sundays there and if they have enough wine they burst into song.
Also try Agrokali Fish Taverna in beautiful San George, 10 minutes drive from Sidari...good for lunch as the dirt track that leads to it has no lighting so not a good idea after a few drinks. The owners name is Kostas and he serves the best seafood on the island....My family and I have been eating there for 10 years!!! Follow the road to the end of the bay and continue down the dirt track that hugs the coast, first Taverna that you reach.
Might see you there.