Some of them do.

If you want to eat in a particular restaurant on a particular night then I would always call in or phone during the day and let them know. You can go to the same place two nights on the trot and one time it will be full and the next empty

The more popular ones can get very busy. Places like The Bistro and The Mermaid always seem full of english regulars so I would reccommend a walk up the beach to book the day before if you are in the area. Fiesta is another popular one. All the others I think you will be fine just walking in, maybe having a drink if you have to wait for a table.