The map above shows the islands along the coast of Tavira and their beaches. The two islands that belong to Tavira are Ilha de Tavira and Ilha de Cabanas de Tavira, and their beaches are (from west to east) Praia do Homem Nu, Praia do Barril, Praia da Terra Estreita, Praia da Ilha de Tavira, and Praia de Cabanas de Tavira.
The beach accessed directly from Tavira (by ferry) is Ilha de Tavira. You can go on a long walk by the sea from there to the other three beaches to the west, or will need to take a ferry or a tourist train from the neighboring villages (you have complete transportation details on the guides to each beach, linked here). You can’t walk to any of the beaches from the center of the city, as they’re all located on islands.